XII. OBSERVATIONS ON THE ANCESTRAL FORM OF THE CHORDATA
The present section of this work would not be complete without some attempt to reconstruct, from the materials recorded in the previous chapters, and from those supplied by comparative anatomy, the characters of the ancestors of the Chordata; and to trace as far as possible from what invertebrate stock this ancestor was derived.
The second of these questions has been recently dealt with in a very suggestive manner by both Dohrn (No. 250) and Semper (Nos. 255 and 256), but it is still so obscure that I shall refrain from any detailed discussion of it.
While differing very widely in many points both Dohrn and Semper have arrived at the view, already tentatively put forward by earlier anatomists, that the nearest allies of the Chordata are to be sought for amongst the Chætopoda, and that the dorsal surface of the Chordata with the spinal cord corresponds morphologically with the ventral surface of the Chætopods with the ventral ganglion chain. In discussing this subject some time ago[100] I suggested that we must look for the ancestors of the Chordata, not in allies of the present Chætopoda, but in a stock of segmented forms descended from the same unsegmented types as the Chætopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally, instead of ventrally to form a median nervous cord. This group of forms, if my suggestion as to its existence is well founded, appears now to have perished. The recent researches of Hubrecht on the anatomy of the Nemertines[101] have, however, added somewhat to the probability of my views, in that they shew that in some existing Nemertines the nerve-cords approach each other very closely in the dorsal line.
[100] Monograph on the development of Elasmobranch Fishes, pp. 170-173.
[101] Hubrecht, "Zur Anat. u. Phys. d. Nervensystems der Nemertinen." Kön. Akad. Wiss. Amsterdam; and "Researches on the Nervous System of Nemertines." Quart. Journ. of Micr. Science, 1880.
With reference to the characters of the ancestor of the Chordata the following pages contain a few tentative suggestions rather than an attempt to deal with the whole subject; while the origin of certain of the organs is dealt with in a more special manner in the chapters on organogeny which form the second part of this work.
Before entering upon the more special subject of this chapter, it will be convenient to clear the ground by insisting on a few morphological conclusions to be drawn from the study of Amphioxus,--a form which, although probably in some respects degenerate, is nevertheless capable of furnishing on certain points very valuable evidence.
(1) In the first place it is clear from Amphioxus that the ancestors of the Chordata were segmented, and that their mesoblast was divided into myotomes which extended even into the region in front of the mouth. The mesoblast of the greater part of what is called the head in the Vertebrata proper was therefore segmented like that of the trunk.
(2) The only internal skeleton present was the unsegmented notochord--a fact which demonstrates that the skeleton is of comparatively little importance for the solution of a large number of fundamental questions, as for example the point which has been mooted recently as to whether gill-clefts existed at one time in front of the present mouth; and for this reason:--that from the evidence of Amphioxus and the lower Vertebrata[102] it is clear that such clefts, if they ever existed, had atrophied completely before the formation of cartilaginous branchial bars; so that any skeletal structures in front of the mouth, which have been interpreted by morphologists as branchial bars, can never have acted in supporting the walls of branchial clefts.
[102] The greater part of the branchial skeleton of Petromyzon appears clearly to belong to an extra-branchial system much more superficially situated than the true branchial bars of the higher forms. At the same time there is no doubt that certain parts of the skeleton of the adult Lamprey have, as pointed out by Huxley, striking points of resemblance to parts of a true mandibular and hyoid arches. Further embryological evidence is required on the subject, but the statements on this head on p. 84 ought to be qualified.
Should Huxley's views on this subject be finally proved correct, it is probable that, taking into consideration the resemblance of these skeletal parts in the Tadpole to those in the Lamprey, the cartilaginous mandibular bar, before being in any way modified to form true jaws, became secondarily adapted to support a suctorial mouth, and that it subsequently became converted into the true jaws. Thus the evolution of this bar in the Frog would be a true repetition of the ancestral history, while its ontogeny in Elasmobranchii and other types would be much abbreviated. For a fuller statement on this point I must refer the reader to the chapter on the skull.
It is difficult to believe that the posterior branchial bars could have coexisted with such a highly developed branchial skeleton as that in Petromyzon, so that the absence of the posterior branchial bars in Petromyzon receives by far its most plausible explanation on the supposition that Petromyzon is descended from a vertebrate stock in which true branchial bars had not been evolved.
(3) The region which, in the Vertebrata, forms the oesophagus and stomach, was, in the ancestors of the Chordata, perforated by gill-clefts. This fact, which has been clearly pointed out by Gegenbaur, is demonstrated by the arrangement of the gill-clefts in Amphioxus, and by the distribution of the vagus nerve in the Vertebrata[103]. On the other hand the insertion of the liver, which was probably a very primitive organ, appears to indicate with approximate certainty the posterior limit of the branchial clefts.
[103] The extension forwards in the vertebrata of an uninterrupted body-cavity into the region previously occupied by visceral clefts presents no difficulty. In Amphioxus the true body cavity extends forwards, more or less divided by the branchial clefts, for the whole length of the branchial region, and in embryos of the lower Vertebrata there is a section of the body cavity--the so-called head-cavities--between each pair of pouches. On the disappearance of the pouches all these parts would naturally coalesce into a continuous whole.
With these few preliminary observations we may pass to the main subject of this section. A fundamental question which presents itself on the threshold of our enquiries is the differentiation of the head.
In the Chætopoda the head is formed of a præoral lobe and of the oral segment; while in Arthropods a somewhat variable number of segments are added behind to this primitive head, and form with it what may be called a secondary compound head. It is fairly clear that the section of the trunk, which, in Amphioxus, is perforated by the visceral clefts, has become the head in the Vertebrates proper, so that the latter forms are provided with a secondary head like that of Arthropods. There remain however difficult questions (1) as to the elements of which this head is composed, and (2) as to the extent of its differentiation in the ancestors of the Chordata.
In Arthropods and Chætopods there is a very distinct element in the head known as the procephalic lobe in the case of Arthropods, and the præoral lobe in that of Chætopods; and this lobe is especially characterized by the fact that the supraoesophageal ganglia and optic organs are formed as differentiations of part of the epiblast covering it. Is such an element to be recognized in the head of the Chordata? From a superficial examination of Amphioxus the answer would undoubtedly be no; but then it has to be borne in mind that Amphioxus, in correlation with its habit of burying itself in sand, is especially degenerate in the development of its sense-organs; so that it is not difficult to believe that its præoral lobe may have become so reduced as not to be recognizable. In the true Vertebrata there is a portion of the head which has undoubtedly many features of the præoral lobe in the types already alluded to, viz. the part containing the cerebral hemispheres and the thalamencephalon. If there is any part of the brain homologous with the supraoesophageal ganglia of the Invertebrates, and it is difficult to believe there is not such a part, it must be part of, or contain, the fore-brain. The fore-brain resembles the supraoesophageal ganglia in being intimately connected in its development with the optic organs, and in supplying with nerves only organs of sense. Its connection with the olfactory organs is an argument in the same direction. Even in Amphioxus there is a small bulb at the end of the nervous tube supplying what is very probably the homologue of the olfactory organ of the Vertebrata; and it is quite possible that this bulb is the reduced rudiment of what forms the fore-brain in the Vertebrata.
The evidence at our disposal appears to me to indicate that the third nerve belongs to the cranio-spinal series of segmental nerves, while the optic and olfactory nerves appear to me equally clearly not to belong to this series[104]. The mid-brain, as giving origin to the third nerve, would appear not to have been part of the ganglion of the præoral lobe.
[104] Marshall, in his valuable paper on the development of the olfactory organ, takes a very different view of this subject. For a discussion of this view I must refer the reader to the chapter on the nervous system.
These considerations indicate with fair probability that the part of the head containing the fore-brain is the equivalent of the præoral lobe of many Invertebrate forms; and the primitive position of the Vertebrate mouth on the ventral side of the head affords a distinct support for this view. It must however be admitted that this part of the head is not sharply separated in development from that behind; and, though the fore-brain is usually differentiated very early as a distinct lobe of the primitive nervous tube, yet that such differentiation is hardly more marked than in the other parts of the brain. The termination of the notochord immediately behind the fore-brain is, however, an argument in favour of the morphological distinctness of the latter structure.
The evidence at our disposal appears to indicate that the posterior part of the head was not differentiated from the trunk in lower Chordata; but that, as the Chordata rose in the scale of development, more and more centralizing work became thrown on the anterior part of the nervous cord, and pari passu this part became differentiated into the mid- and hind-brain. An analogy for such a differentiation is supplied in the compound suboesophageal ganglion of many Arthropods; and, as will be shewn in the chapter on the nervous system, there is strong embryological evidence that the mid- and hind-brains had primitively the same structure as the spinal cord. The head appears however to have suffered in the course of its differentiation a great concentration in its posterior part, which becomes progressively more marked, even within the limits of the surviving Vertebrata. This concentration is especially shewn in the structure of the vagus nerve, which, as first pointed out by Gegenbaur, bears evidence of having been originally composed of a great series of nerves, each supplying a visceral cleft. Rudiments of the posterior nerves still remain as the branches to the oesophagus and stomach[105].
[105] The lateral branch of the vagus nerve probably became differentiated in connection with the lateral line, which seems to have been first formed in the head, and subsequently to have extended into the trunk (vide section on Lateral Line).
The atrophy of the posterior visceral clefts seems to have taken place simultaneously with the concentration of the neural part of the head; but the former process did not proceed so rapidly as the latter, so that the visceral region of the head is longer in the lower Vertebrata than the neural region, and is dorsally overlapped by the anterior part of the spinal cord and the anterior muscle-plates (vide fig. 47).
On the above view the posterior part of the head must have been originally composed of a series of somites like those of the trunk, but in existing Vertebrata all trace of these, except in so far as they are indicated by the visceral clefts, has vanished in the adult. The cranial nerves however, especially in the embryo, still indicate the number of anterior somites; and an embryonic segmentation of the mesoblast has also been found in many lower forms in the region of the head, giving rise to a series of cavities known as head-cavities, enclosed by mesoblastic walls which afterwards break up into muscles. These cavities correspond with the nerves, and it appears that there is a præmandibular cavity corresponding with the third nerve (fig. 193, 1pp) and a mandibular cavity (2pp) and a cavity in each of the succeeding visceral arches. The fifth nerve, the seventh nerve, the glossopharyngeal nerve, and the successive elements of the vagus nerve correspond with the posterior head-cavities.
[FIG. 193. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO.
The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the præmandibular and mandibular head-cavities 1pp and 2pp, etc.
fb. fore-brain; l. lens of eye; m. mouth; pt. upper end of mouth, forming pituitary involution; 1ao. mandibular aortic arch; 1pp. and 2pp. first and second head-cavities; 1vc. first visceral cleft; V. fifth nerve; aun. ganglion of auditory nerve; VII. seventh nerve; aa. dorsal aorta; acv. anterior cardinal vein; ch. notochord.]
The medullary canal. The general history of the medullary plate seems to point to the conclusion that the central canal of the nervous system has been formed by a groove having appeared in the ancestor of the Chordata along the median dorsal line, which caused the sides of the nervous plate, which was placed immediately below the skin, or may perhaps at that stage not have been distinctly differentiated from the skin, to be bent upwards; and that this groove subsequently became converted into a canal. This view is not only supported by the actual development of the central canal of the nervous system (the types of Teleostei, Lepidosteus and Petromyzon being undoubtedly secondary), but also (1) by the presence of cilia in the epithelium lining the canal, probably inherited from cilia coating the external skin, and (2) by the posterior roots arising from the extreme dorsal line (fig. 194), a position which can most easily be explained on the supposition that the two sides of the plate, from which the nerves originally proceeded have been folded up so as to meet each other in the median dorsal line[106].
[106] Vide for further details the chapter on the nervous system.
The medullary plate, before becoming folded to form the medullary groove, is (except in Amphibia) without any indication of being composed of two halves. In both the embryo and adult the walls of the tube have however a structure which points to their having arisen from the coalescence of two lateral, and most probably at one time independent, cords; and as already indicated this is the view I am myself inclined to adopt; vide pp. 303 and 304.
[FIG. 194. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E.
nc. neural canal; pr. posterior root of spinal nerve; x. subnotochordal rod; ao. aorta; sc. somatic mesoblast; sp. splanchnic mesoblast; mp. muscle-plate; mp´. portion of muscle-plate converted into muscle; Vv. portion of the vertebral plate which will give rise to the vertebral bodies; al. alimentary tract.]
The origin and nature of the mouth. The most obvious point connected with the development of the mouth is the fact that in all vertebrate embryos it is placed ventrally, at some little distance from the front end of the body. This feature is retained in the adult stage in Elasmobranchii, the Myxinoids, and some Ganoids, but is lost in other vertebrate forms. A mouth, situated as is the embryonic vertebrate mouth, is very ill adapted for biting; and though it acquires in this position a distinctly biting character in the Elasmobranchii, yet it is almost certain that it had not such a character in the ancestral Chordata, and that its terminal position in higher types indicates a step in advance of the Elasmobranchii.
On the structure of the primitive mouth there appears to me to be some interesting embryological evidence, to which attention has already been called in the preceding chapters. In a large number of the larvæ or embryos of the lower Vertebrates the mouth has a more or less distinctly suctorial character, and is connected with suctorial organs which may be placed either in front of or behind it. The more important instances of this kind are (1) the Tadpoles of the Anura, with their posteriorly placed suctorial disc, (2) Lepidosteus larva (fig. 195) with its anteriorly placed suctorial disc, (3) the adhesive papillæ of the larvæ of the Tunicata. To these may be added the suctorial mouth of the Myxinoid fishes[107].
[107] The existing Myxinoid Fishes are no doubt degenerate types, as was first clearly pointed out by Dohrn; but at the same time (although Dohrn does not share this view) it appears to me almost certain that they are the remnants of a large and very primitive group, which have very likely been preserved owing to their parasitic or semiparasitic habits; much in the same way as many of the Insectivora have been preserved owing to their subterranean habits. I am acquainted with no evidence, embryological or otherwise, that they are degraded gnathostomatous forms, and the group probably disappeared as a whole from its incapacity to compete successfully with Vertebrata in which true jaws had become developed.
All these considerations point to the conclusion that in the ancestral Chordata the mouth had a more or less definitely suctorial character[108], and was placed on the ventral surface immediately behind the præoral lobe; and that this mouth has become in the higher types gradually modified for biting purposes, and has been carried to the front end of the head.
[108] I do not conceive that the existence of suctorial structures necessarily implies parasitic habits. They might be used for various purposes, especially by predaceous forms not provided with jaws.
The mouth in Elasmobranchii and other Vertebrates is originally a wide somewhat rhomboidal cavity (fig. 28 G); on the development of the mandibular and its maxillary (pterygo-quadrate) process the opening of the mouth becomes narrowed to a slit. The wide condition of the mouth may not improbably be interpreted as a remnant of the suctorial state. The fact that no more definite remnants of the suctorial mouth are found in so primitive a group as the Elasmobranchii is probably to be explained by the fact that the members of this group undergo an abbreviated development within the egg.
While the embryological data appear to me to point to the existence of a primitive suctorial mouth, very different conclusions have been put forward by other embryologists, more especially by Dohrn, which are sufficiently striking and suggestive to merit a further discussion.
As mentioned above, both Dohrn and Semper hold that the Vertebrata are descended from Chætopod-like forms, in which the ventral surface has become the dorsal. In consequence of this view Dohrn has arrived at the following conclusions: (1) that primitively the alimentary canal perforated the nervous system in the region of the original oesophageal nerve-ring; (2) that there was therefore an original dorsal mouth (the present ventral mouth of the Chætopoda); and (3) that the present mouth was secondary and derived from two visceral clefts which have ventrally coalesced.
[FIG. 195. VENTRAL VIEW OF THE HEAD OF A LEPIDOSTEUS EMBRYO SHORTLY BEFORE HATCHING, TO SHEW THE LARGE SUCTORIAL DISC.
m. mouth; op. eye; sd. suctorial disc.]
A full discussion of these views[109] is not within the scope of this work; but, while recognizing that there is much to be said in favour of the interchange of the dorsal and ventral surfaces, I am still inclined to hold that the difficulties involved in this view are so great that it must, provisionally at least, be rejected; and that there are therefore no reasons against supposing the present vertebrate mouth to be the primitive mouth. There is no embryological evidence in favour of the view adopted by Dohrn that the present mouth was formed by the coalescence of two clefts.
[109] For a partial discussion of this subject I would refer the reader to my Monograph on Elasmobranch Fishes, pp. 165-172.
If it is once admitted that the present mouth is the primitive mouth, and is more or less nearly in its original situation, very strong evidence will be required to shew that any structures originally situated in front of it are the remnants of visceral clefts; and if it should be proved that such remnants of visceral clefts were present, the views so far arrived at in this section would, I think, have to be to a large extent reconsidered.
The nasal pits have been supposed by Dohrn to be remnants of visceral clefts, and this view has been maintained in a very able manner by Marshall. The arguments of Marshall do not, however, appear to me to have any great weight unless it is previously granted that there is an antecedent probability in favour of the presence of a pair of gill-clefts in the position of the nasal pits; and even then the development of the nasal pits as epiblastic involutions, instead of hypoblastic outgrowths, is a serious difficulty which has not in my opinion been successfully met. A further argument of Marshall from the supposed segmental nature of the olfactory nerve has already been spoken of.
While most of the structures supposed to be remains of gill-clefts in front of the mouth do not appear to me to be of this nature, there is one organ which stands in a more doubtful category. This organ is the so-called choroid gland. The similarity of this organ to the pseudobranch of the mandibular or hyoid arch was pointed out to me by Dohrn, and the suggestion was made by him that it is the remnant of a præmandibular gill which has been retained owing to its functional connection with the eye[110]. Admitting this explanation to be true (which however is by no means certain) are we necessarily compelled to hold that the choroid gland is the remnant of a gill-cleft originally situated in front of the mouth? I believe not. It is easy to conceive that there may originally have been a præmandibular cleft behind the suctorial mouth, but that this cleft gradually atrophied (for the same reasons that the mandibular cleft shews a tendency to atrophy in existing fishes, &c.), the rudiment of the gill (choroid gland) alone remaining to mark its situation. After the disappearance of this cleft the suctorial mouth may have become relatively shifted backwards. In the meantime the branchial bars became developed, and as the mouth was changed into a biting one, the bar (the mandibular arch) supporting the then first cleft became gradually modified and converted into a supporting apparatus for the mouth, and finally formed the skeleton of the jaws. In the hyostylic Vertebrata the hyoid arch also became modified in connection with the formation of the jaws.
[110] The probability of the choroid gland having the meaning attributed to it by Dohrn is strengthened by the existence of a præmandibular segment as evidenced by the presence of a præmandibular head-cavity, the walls of which as shewn by Marshall and myself give rise to the majority of the eye-muscles and of a nerve (the third nerve, cf. Marshall) corresponding to it; so that these parts together with the choroid gland may be rudiments belonging to the same segment. On the other hand the absence of the choroid gland in Ganoidei and Elasmobranchii, where a mandibular pseudobranch is present, coupled with the absence of a mandibular pseudobranch in Teleostei where alone a choroid gland is present, renders the above view about the choroid gland somewhat doubtful. A thorough investigation of the ontogeny of the choroid gland might throw further light on this interesting question, but I think it not impossible that the choroid gland may be nothing else but the modified mandibular pseudobranch, a view which fits in very well with the relations of the vessels of the Elasmobranch mandibular pseudobranch to the choroid. For the relations and structure of the choroid gland vide F. Müller, Vergl. Anat. Myxinoiden, Part III. p. 82.
It is possible that the fourth nerve and the superior oblique muscle of the eye which it supplies may be the last remaining remnants of a second præmandibular segment originally situated between the segment of the third nerve and that of the fifth nerve (mandibular segment).
The conclusions arrived at may be summed up as follows:
The relations which exist in all jaw-bearing Vertebrates between the mandibular arch and the oral aperture are secondary, and arose pari passu with the evolution of the jaws[111].
[111] I do not mean to exclude the possibility of the mandibular arch having supported a suctorial mouth before it became converted into a pair of jaws.
[FIG. 196. THE HEADS OF ELASMOBRANCH EMBRYOS AT TWO STAGES VIEWED AS TRANSPARENT OBJECTS.
A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo.
III. third nerve; V. fifth nerve; VII. seventh nerve; au.n. auditory nerve; gl. glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland; mb. mid-brain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op. eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; Vc. visceral clefts; eg. external gills; pp. sections of body cavity in the head.]
The cranial flexure and the form of the head in vertebrate embryos. All embryologists who have studied the embryos of the various vertebrate groups have been struck with the remarkable similarity which exists between them, more especially as concerns the form of the head. This similarity is closest between the members of the Amniota, but there is also a very marked resemblance between the Amniota and the Elasmobranchii. The peculiarity in question, which is characteristically shewn in fig. 196, consists in the cerebral hemispheres and thalamencephalon being ventrally flexed to such an extent that the mid-brain forms the termination of the long axis of the body. At a later period in development the cerebral hemispheres come to be placed at the front end of the head; but the original nick or bend of the floor of the brain is never got rid of.
It is obvious that in dealing with the light thrown by embryology on the ancestral form of the Chordata the significance of this peculiar character of the head of many vertebrate embryos must be discussed. Is the constancy of this character to be explained by supposing that at one period vertebrate ancestors had a head with the same features as the embryonic head of existing Vertebrata?
This is the most obvious explanation, but it does not at the same time appear to me satisfactory. In the first place the mouth is so situated at the time of the maximum cranial flexure that it could hardly have been functional; so that it is almost impossible to believe that an animal with a head such as that of these embryos can have existed.
Then again, this type of embryonic head is especially characteristic of the Amniota, all of which are developed in the egg. It is not generally so marked in the Ichthyopsida. In Amphibia, Teleostei, Ganoidæ and Petromyzontidæ, the head never completely acquires the peculiar characteristic form of the head of the Amniota, and all these forms are hatched at a relatively much earlier phase of development, so that they are leading a free existence at a stage when the embryos of the Amniota are not yet hatched. The only Ichthyopsidan type with a head like that of the Amniota is the Elasmobranchii, and the Elasmobranchii are the only Ichthyopsida which undergo the major part of their development within the egg.
These considerations appear to shew that the peculiar characters of the embryonic head above alluded to are in some way connected with an embryonic as opposed to a larval development; and for reasons which are explained in the section on larval forms, it is probable that a larval development is a more faithful record of ancestral history than an embryonic development. The flexure at the base of the brain appears however to be a typical vertebrate character, but this flexure never led to a conformation of the head in the adult state similar to that of the embryos of the Amniota. The form of the head in these embryos is probably to be explained by supposing that some advantage is gained by a relatively early development of the brain, which appears to be its proximate cause; and since these embryos had not to lead a free existence (for which such a form of the head would have been unsuited) there was nothing to interfere with the action of natural selection in bringing about this form of head during foetal life.
Postanal gut and neurenteric canal. One of the most remarkable structures in the trunk is the postanal gut (fig. 197). Its structure is fully dealt with in the chapter on the alimentary tract, but attention may here be called to the light which it appears to throw on the characters of the ancestor of the Chordata.
In face of the facts which are known with reference to the postanal section of the alimentary tract, it can hardly be doubted that this portion of the alimentary tract must have been at one time functional. This seems to me to be shewn (1) by the constancy and persistence of this obviously now functionless rudiment, (2) by its greater development in the lower than in the higher forms, (3) by its relation to the formation of the notochord and subnotochordal rod.
[FIG. 197. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOMBINATOR. (After Götte.)
m. mouth; an. anus; l. liver; ne. neurenteric canal; mc. medullary canal; ch. notochord; pn. pineal gland.]
If the above position be admitted, it is not permissible to shirk the conclusions which seem necessarily to follow, however great the difficulties may be which are involved in their acceptance. These conclusions have in part already been dealt with by Dohrn in his suggestive tract (No. 250). In the first place the alimentary canal must primitively have been continued to the end of the tail; and if so, it is hardly credible that the existing anus can have been the original one. Although, therefore, it is far from easy, on the physiological principles involved in the Darwinian theory, to understand the formation of a new anus[112]; it is nevertheless necessary to believe that the present vertebrate anus is a formation acquired within the group of the Chordata, and not inherited from some older group. This involves a series of further consequences. The opening of the urinogenital ducts into the cloaca must also be secondary, and it is probable that the segmental tubes were primitively continued along the whole postanal region of the vertebrate tail, opening into the body cavity which embryology proves to have been originally present there. They are in fact continued in many existing forms for some distance behind the present anus. If the present anus is secondary, there must have been a primitive anus, which was probably situated behind the postanal vesicle; and therefore in the region of the neurenteric canal. The neurenteric canal is, however, the remnant of the blastopore (vide p. 277). It follows, therefore, that the vertebrate blastopore is probably almost, if not exactly identical in position with the primitive anus. This consideration may assist in explaining the remarkable phenomenon of the existence of the neurenteric canal. The attempt has already been made to shew that the central canal of the nervous system is really a groove converted into a tube and lined by the external epidermis. This tube (as may be concluded from embryological considerations) was probably at first open posteriorly, and no doubt terminated at the primitive anus. On the closure of the primitive anal opening, the terminal portions of the postanal gut and the neural tube, may conceivably have been so placed that both of them opened into a common cavity, which previously had communication with the exterior by the anus. Such an arrangement would necessarily result in the formation of a neurenteric canal. It seems not impossible that a dilated vesicle, often present at the end of the postanal gut (vide fig. 28*, p. 58), may have been the common cavity into which both neural and alimentary tubes opened[113]. Till further light is thrown by fresh discoveries upon the primitive condition of the posterior continuation of the vertebrate alimentary tract, it is perhaps fruitless to attempt to work out more in detail the above speculation.
[112] Dohrn (No. 250, p. 25) gives an explanation of the origin of the new anus which does not appear to me quite satisfactory.
[113] As pointed out in Vol. II. p. 255, there is a striking similarity between the history of the neurenteric canal in Vertebrates, and the history of the blastopore and ventral groove as described by Kowalevsky in the larva of Chiton. Mr A. Sedgwick has pointed out to me that the ciliated ventral groove in Protoneomenia, which contains the anus, is probably the homologue of the groove found in the larva of Chiton, and not, as usually supposed, simply the foot. Were this groove to be converted into a canal, on the sides of which were placed the nervous cords, there would be formed a precisely similar neurenteric canal to that in Vertebrata, though I do not mean to suggest that there is any homology between the two (vide Hubrecht, Zool. Anzeiger, 1880, p. 589).
Body cavity and mesoblastic somites. The Chordata, or at least the most primitive existing members of the group, are characterized by the fact that the body cavity arises as a pair of outgrowths of the archenteric cavity. This feature[114] in the development is a nearly certain indication that the Chordata are a very primitive stock. The most remarkable point with reference to the development of the two outgrowths is, however, the fact that the dorsal part of each outgrowth becomes separated from the ventral. Its walls become segmented and form the mesoblastic somites, which eventually, on the obliteration of their cavity, give rise to the muscle-plates and to the tissue surrounding the notochord. It is not easy to understand the full significance of the processes concerned in the formation of the mesoblastic somites (vide p. 296). The mesoblastic somites have no doubt a striking resemblance to the mesoblastic somites of the Chætopods, and most probably the segmentation of the mesoblast in the two groups is a phenomenon of the same nature; but the difference in origin between the two types of mesoblastic somites is so striking, and the development of the muscular system from them is so dissimilar in the two groups, as to render a direct descent of the Chordata from the Chætopoda very improbable. The ventral parts of the original outgrowth give rise to the permanent body cavity, which appears originally to have been divided into two parts by a dorsal and a ventral mesentery.
[114] Vide the chapter on the Germinal Layers.
The notochord. The most characteristic organ of the Chordata is without doubt the notochord. The ontogenetic development of this organ probably indicates that it arose as a differentiation of the dorsal wall of the archenteron; at the same time it is not perhaps safe to lay too much stress upon its mode of development. Embryological and anatomical evidence demonstrate, however, in the clearest manner that the early Chordata were provided with this organ as their sole axial skeleton; and no invertebrate group can fairly be regarded as genetically related to the Chordata till it can be shewn to possess some organ either derived from a notochord, or capable of having become developed into a notochord. No such organ has as yet been recognized in any invertebrate group[115].
[115] In the Chætopods various organs have been interpreted as rudiments of a notochord, but none of these interpretations will bear examination.
Gill-clefts. The gill-clefts, which are essentially pouches of the throat opening externally, constitute extremely characteristic organs of the Chordata, and have always been taken into consideration in any comparison between the Chordata and the Invertebrata.
Amongst the Invertebrata organs of undoubtedly the same nature are, so far as I know, only found in Balanoglossus, where they were discovered by Kowalevsky. The resemblance in this case is very striking; but although it is quite possible that the gill-clefts in Balanoglossus are genetically connected with those of the Chordata, yet the organization of Balanoglossus is as a whole so different from that of the Chordata that no comparison can be instituted between the two groups in the present state of our knowledge.
Other organs of the Invertebrata have some resemblance to the gill-clefts. The lateral pits of the Nemertines, which appear to grow out as a pair of oesophageal diverticula, which are eventually placed in communication with the exterior by a pair of ciliated canals (vide Vol. II. pp. 200 and 202), are such organs.
Semper (No. 256) has made the interesting discovery that in the budding of Nais and Chætogaster two lateral masses of cells, in each of which a lumen may be formed, unite with the oral invagination and primitive alimentary canal to form the permanent cephalic gut. The lateral masses of cells are regarded by him as branchial passages homologous in some way with those in the Chordata. The somewhat scanty observations on this subject which he has recorded do not appear to me to lend much support to this interpretation.
It is probable that the part of the alimentary tract in which gill-clefts are present was originally a simple unperforated tube provided with highly vascular walls; and that respiration was carried on in it by the alternate introduction and expulsion of sea water. A more or less similar mode of respiration has been recently shewn by Eisig[116] to take place in the fore part of the alimentary tract of many Chætopods. This part of the alimentary tract was probably provided with paired cæcal pouches with their blind ends in contiguity with the skin.
[116] "Ueb. d. Vorkommen eines schwimmblasenähnlichen Organs bei Anneliden." Mittheil. a. d. zool. Station zu Neapel, Vol. II. 1881.
Perforations placing these pouches in communication with the exterior must be supposed to have been formed; and the existence of openings into the alimentary tract at the end of the tentacles of many Actiniæ and of the hepatic diverticula of some nudibranchiate Molluscs (Eolis, &c.[117]) shews that such perforations may easily be made. On the formation of such perforations the water taken in at the mouth would pass out by them; and the respiration would be localized in the walls of the pouches leading to them, and thus the typical mode of respiration of the Chordata would be established.
[117] The openings of the hepatic diverticula through the sacks lined with thread cells are described by Hancock and Embleton, Ann. and Mag. of Nat. History, Vol. XV. 1845, p. 82. Von Jhering has also recently described these openings (Zool. Anzeiger, No. 23) and apparently attributes their discovery to himself.
Phylogeny of the Chordata. It may be convenient to shew in a definite way the bearing of the above speculations on the phylogeny of the Chordata. For this purpose, I have drawn up the subjoined table, which exhibits what I believe to be the relationships of the existing groups of the Chordata. Such a table cannot of course be constructed from embryological data alone, and it does not fall within the scope of this work to defend its parts in detail.
MAMMALIA SAUROPSIDA | | ----------------------------- | PROTO-AMNIOTA AMPHIBIA | | ---------------------------------- | TELEOSTEI PROTO-PENTADACTYLOIDEI | | GANOIDEI | ---- DIPNOI | | --------------------| PROTO-GANOIDEI | |-- HOLOCEPHALI |------- ELASMOBRANCHII | PROTO-GNATHOSTOMATA | --------------------------| | | Cyclostomata PROTO-VERTEBRATA | --------------------------|------------------------- | | Cephalochorda PROTOCHORDATA Urochorda
In the above table the names printed in large capitals are hypothetical groups. The other groups are all in existence at the present day, but those printed in Italics are probably degenerate.
The ancestral forms of the Chordata, which may be called the Protochordata, must be supposed to have had (1) a notochord as their sole axial skeleton, (2) a ventral mouth, surrounded by suctorial structures, and (3) very numerous gill-slits. Two degenerate offshoots of this stock still persist in Amphioxus (Cephalochorda), and the Ascidians (Urochorda).
The direct descendants of the ancestral Chordata, were probably a group which may be called the Protovertebrata, of which there is no persisting representative. In this group, imperfect neural arches were probably present; and a ventral suctorial mouth without a mandible and maxillæ was still persistent. The branchial clefts had, however, become reduced in number, and were provided with gill-folds; and a secondary head (vide p. 313), with brain and organs of sense like those of the higher Vertebrata, had become formed.
The Cyclostomata are probably a degenerate offshoot of this group.
With the development of the branchial bars, and the conversion of the mandibular bar into the skeleton of the jaws, we come to the Proto-gnathostomata. The nearest living representatives of this group are the Elasmobranchii, which still retain in the adult state the ventrally placed mouth. Owing to the development of food-yolk in the Elasmobranch ovum the early stages of development are to some extent abbreviated, and almost all trace of a stage with a suctorial mouth has become lost.
We next come to an hypothetical group which we may call the Proto-ganoidei. Bridge, in his memoir on Polyodon[118], which contains some very interesting speculations on the affinities of the Ganoids, has called this group the Pneumatocoela, from the fact that we find for the first time a full development of the air-bladder, though it is possible that a rudiment of this organ, in the form of a pouch opening on the dorsal side of the stomachic extremity of the oesophagus, was present in the earlier type.
[118] Phil. Trans. 1878. Part II.
Existing Ganoids are descendants of the Proto-ganoidei. Some of them at all events retain in larval life the suctorial mouth of the Protovertebrata; and the mode of formation of their germinal layers, resembling as it does that in the Lamprey and the Amphibia, probably indicates that they are not descended from forms with a large food-yolk like that of Elasmobranchii, and that the latter group is therefore a lateral offshoot from the main line of descent.
Of the two groups into which the Ganoidei may be divided it is clear that certain members of the one (Telcostoidei), viz. Lepidosteus and Amia, shew approximations to the Teleostei, which no doubt originated from the Ganoids; while the other (Selachoidei or Sturiones) is more nearly related to the Dipnoi. Polypterus has also marked affinities in this direction, e.g. the external gills of the larva (vide p. 118).
The Teleostei, which have in common a meroblastic segmentation, had probably a Ganoid ancestor, the ova of which were provided with a large amount of food-yolk. In most existing Teleostei, the ovum has become again reduced in size, but the meroblastic segmentation has been preserved. It is quite possible that Amia may also be a descendant of the Ganoid ancestor of the Teleostei; but Lepidosteus, as shewn by its complete segmentation, is clearly not so.
The Dipnoi as well as all the higher Vertebrata are descendants of the Proto-ganoidei.
The character of the limbs of higher Vertebrata indicates that there was an ancestral group, which may be called the Proto-pentadactyloidei, in which the pentadactyle limb became established; and that to this group the common ancestor of the Amphibia and Amniota belonged.
It is possible that the Plesiosauri and Ichthyosauri of Mesozoic times may have been more nearly related to this group than either to the Amniota or the Amphibia. The Proto-pentadactyloidei were probably much more closely related to the Amphibia than to the Amniota. They certainly must have been capable of living in water as well as on land, and had of course persistent branchial clefts. It is also fairly certain that they were not provided with large-yolked ova, otherwise the mode of formation of the layers in Amphibia could not be easily explained.
The Mammalia and Sauropsida are probably independent offshoots from a common stem which may be called the Protoamniota.
BIBLIOGRAPHY.
(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes. London, 1878.
(250) A. Dohrn. Der Ursprung d. Wirbelthiere und d. Princip. d. Functionswechsel. Leipzig, 1875.
(251) E. Haeckel. Schöpfungsgeschichte. Leipzig. Vide also Translation. The History of Creation. King and Co., London, 1876.
(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Anthropogeny. Kegan Paul and Co., London, 1878.
(253) A. Kowalevsky. "Entwicklungsgeschichte d. Amphioxus lanceolatus." Mém. Acad. d. Scien. St Pétersbourg, Ser. VII. Tom. XI. 1867, and Archiv f. mikr. Anat., Vol. XIII. 1877.
(254) A. Kowalevsky. "Weitere Stud. üb. d. Entwick. d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871.
(255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen." Arbeit. a. d. zool.-zoot. Instit. Würzburg, Vol. II. 1875.
(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere." Arbeit. a. d. zool.-zoot. Instit. Würzburg, Vol. III. 1876-1877.
XIII. GENERAL CONCLUSIONS
I. THE MODE OF ORIGIN AND HOMOLOGIES OF THE GERMINAL LAYERS.
It has already been shewn in the earlier chapters of the work that during the first phases of development the history of all the Metazoa is the same. They all originate from the coalescence of two cells, the ovum and spermatozoon. The coalesced product of these cells--the fertilized ovum--then undergoes a process known as the segmentation, in the course of which it becomes divided in typical cases into a number of uniform cells. An attempt was made from the point of view of evolution to explain these processes. The ovum and spermatozoon were regarded as representing phylogenetically two physiologically differentiated forms of a Protozoon; their coalescence was equivalent to conjugation: the subsequent segmentation of the fertilized ovum was the multiplication by division of the organism resulting from the conjugation; the resulting organisms, remaining, however, united to form a fresh organism in a higher state of aggregation.
In the systematic section of this work the embryological history of the Metazoa has been treated. The present chapter contains a review of the cardinal features of the various histories, together with an attempt to determine how far there are any points common to the whole of these histories; and the phylogenetic interpretation to be given to such points.
Some years ago it appeared probable that a definite answer would be given to the questions which must necessarily be raised in the present chapter; but the results of the extended investigations made during the last few years have shewn that these expectations were premature, and in spite of the numerous recent valuable contributions to this branch of Embryology, amongst which special attention may be called to those of Kowalevsky (No. 277), Lankester (Nos. 278 and 279), and Haeckel (No. 266), there are few embryologists who would venture to assert that any answers which can be given are more than tentative gropings towards the truth.
In the following pages I aim more at summarising the facts, and critically examining the different theories which can be held, than at dogmatically supporting any definite views of my own.
In all the Metazoa, the development of which has been investigated, the first process of differentiation, which follows upon the segmentation, consists in the cells of the organism becoming divided into two groups or layers, known respectively as epiblast and hypoblast.
These two layers were first discovered in the young embryos of vertebrated animals by Pander and Von Baer, and have been since known as the germinal layers, though their cellular nature was not at first recognised. They were shewn, together with a third layer, or mesoblast, which subsequently appears between them, to bear throughout the Vertebrata constant relations to the organs which became developed from them. A very great step was subsequently made by Remak (No. 287), who successfully worked out the problem of vertebrate embryology on the cellular theory.
Rathke in his memoir on the development of Astacus (No. 286) attempted at a very early period to extend the doctrine of the derivation of the organs from the germinal layers to the Invertebrata. In 1859 Huxley made an important step towards the explanation of the nature of these layers by comparing them with the ectoderm and endoderm of the Hydrozoa; while the brilliant researches of Kowalevsky on the development of a great variety of invertebrate forms formed the starting point of the current views on this subject.
The differentiation of the epiblast and hypoblast may commence during the later phases of the segmentation, but is generally not completed till after its termination. Not only do the cells of the blastoderm become differentiated into two layers, but these two layers, in the case of a very large number of ova with but little food-yolk, constitute a double-walled sack--the gastrula (fig. 198)--the characters of which are too well known to require further description. Following the lines of phylogenetic speculation above indicated, it may be concluded that the two-layered condition of the organism represents in a general way the passage from the protozoon to the metazoon condition. It is probable that we may safely go further, and assert that the gastrula reproduces, with more or less fidelity, a stage in the evolution of the Metazoa, permanent in the simpler Hydrozoa, during which the organism was provided with (1) a fully developed digestive cavity (fig. 198 b) lined by the hypoblast with digestive and assimilative functions, (2) an oral opening (a), and (3) a superficial epiblast (d). These generalisations, which are now widely accepted, are no doubt very valuable, but they leave unanswered the following important questions:
(1) By what steps did the compound Protozoon become differentiated into a Metazoon? (2) Are there any grounds for thinking that there is more than one line along which the Metazoa have become independently evolved from the Protozoa? (3) To what extent is there a complete homology between the two primary germinal layers throughout the Metazoa?
[FIG. 198. DIAGRAM OF A GASTRULA. (From Gegenbaur.)
a. mouth; b. archenteron; c. hypoblast; d. epiblast.]
Ontogenetically there is a great variety of processes by which the passage from the segmented ovum to the two-layered or diploblastic condition is arrived at.
These processes may be grouped under the following heads:
1. Invagination. Under this term a considerable number of closely connected processes are included. When the segmentation results in the formation of a blastosphere, one half of the blastosphere may be pushed in towards the opposite half, and a gastrula be thus produced (fig. 199, A and B). This process is known as embolic invagination. Another process, known as epibolic invagination, consists in epiblast cells growing round and enclosing the hypoblast (fig. 200). This process replaces the former process when the hypoblast cells are so bulky from being distended by food-yolk that their invagination is mechanically impossible.
[FIG. 199. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA, VIEWED IN OPTICAL SECTION. (After Selenka.)
A. Stage at the close of segmentation. B. Gastrula stage.
mr. micropyle; fl. chorion; s.c. segmentation cavity; bl. blastoderm; ep. epiblast; hy. hypoblast; ms. amoeboid cells derived from hypoblast; a.e. archenteron.]
There are various peculiar modifications of invagination which cannot be dealt with in detail.
Invagination in one form or other occurs in some or all the members of the following groups:
[FIG. 200. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN EARLY STAGE OF DEVELOPMENT, TO SHEW THE NATURE OF EPIBOLIC INVAGINATION. (After Kowalevsky.)
ep. epiblast; ms. mesoblastic band; hy. hypoblast.]
The Dicyemidæ, Calcispongiæ (after the amphiblastula stage) and Silicispongiæ, Coelenterata, Turbellaria, Nemertea, Rotifera, Mollusca, Polyzoa, Brachiopoda, Chætopoda, Discophora, Gephyrea, Chætognatha, Nematelminthes, Crustacea, Echinodermata, and Chordata.
The gastrula of the Crustacea is peculiar, as is also that of many of the Chordata (Reptilia, Aves, Mammalia), but there is every reason to suppose that the gastrulæ of these groups are simply modifications of the normal type.
2. Delamination. Three types of delamination may be distinguished:
a. Delamination where the cells of a solid morula become divided into a superficial epiblast, and a central solid mass in which the digestive cavity is subsequently hollowed out (fig. 201).
[FIG. 201. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM, TO ILLUSTRATE THE FORMATION OF THE LAYERS BY DELAMINATION. (After Metschnikoff.)
A. Stage after the delamination; ep. epiblastic invagination to form pneumatocyst.
B. Later stage after the formation of the gastric cavity in the solid hypoblast. po. polypite; t. tentacle; pp. pneumatocyst; ep. epiblast of pneumatocyst; hy. hypoblast surrounding pneumatocyst.]
b. Delamination where the segmented ovum has the form of a blastosphere, the cells of which give rise by budding to scattered cells in the interior of the vesicle, which, though they may at first form a solid mass, finally arrange themselves in the form of a definite layer around a central digestive cavity (fig. 202).
c. Delamination where the segmented ovum has the form of a blastosphere in the cells of which the protoplasm is differentiated into an inner and an outer part. By a subsequent process the inner parts of the cells become separated from the outer, and the walls of the blastosphere are so divided into two distinct layers (fig. 205).
Although the third of these processes is usually regarded as the type of delamination, it does not, so far as I know, occur in nature, but is most nearly approached in Geryonia (fig. 203).
The first type of delamination is found in the Ceratospongiæ, some Silicispongiæ (?), and in many Hydrozoa and Actinozoa, and in Nemertea and Nematelminthes (Gordioidea?). The second type occurs in many Porifera [Calcispongiæ (Ascetta), Myxospongiæ], and in some Coelenterata, and Brachiopoda (Thecidium).
[FIG. 202. THREE LARVAL STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.)
A. Blastosphere stage with hypoblast spheres becoming budded off into central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity. ep. epiblast; hy. hypoblast; al. gastric cavity.]
Delamination and invagination are undoubtedly the two most frequent modes in which the layers are differentiated, but there are in addition several others. In the first place the whole of the Tracheata (with the apparent exception of the Scorpion) develop, so far as is known, on a plan peculiar to them, which approaches delamination. This consists in the appearance of a superficial layer of cells enclosing a central yolk mass, which corresponds to the hypoblast (figs. 204 and 214). This mode of development might be classed under delamination, were it not for the fact that the early development of many Crustacea is almost the same, but is subsequently followed by an invagination (fig. 208), which apparently corresponds to the normal invagination of other types. There are strong grounds for thinking that the tracheate type of formation of the epiblast and hypoblast is a secondary modification of an invaginate type (vide Vol. II. p. 457).
[FIG. 203. DIAGRAMMATIC FIGURES SHEWING THE DELAMINATION OF THE EMBRYO OF GERYONIA. (After Fol.)
A. Stage at the commencement of the delamination; the dotted lines x shew the course of the next planes of division. B. Stage at the close of the delamination. cs. segmentation cavity; a. endoplasm; b. ectoplasm; ep. epiblast; hy. hypoblast.]
[FIG. 204. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER. (After Metschnikoff.)
In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (bl) which form a blastoderm enveloping the large yolk-spheres. In C the blastoderm has become divided into two layers.]
The type of some Turbellaria (Stylochopsis ponticus) and that of Nephelis amongst the Discophora is not capable of being reduced to the invaginate type.
The development of almost all the parasitic groups, i.e. the Trematoda, the Cestoda, the Acanthocephala, and the Linguatulida, and also of the Tardigrada, Pycnogonida, and other minor groups, is too imperfectly known to be classed with either the delaminate or invaginate types.
It will, I think, be conceded on all sides that, if any of the ontogenetic processes by which a gastrula form is reached are repetitions of the process by which a simple two-layered gastrula was actually evolved from a compound Protozoon, these processes are most probably of the nature either of invagination or of delamination.
The much disputed questions which have been raised about the gastrula and planula theories, originally put forward by Haeckel and Lankester, resolve themselves then into the simple question, whether any, and if so which, of the ontogenetic processes by which the gastrula is formed are repetitions of the phylogenetic origin of the gastrula.
It is very difficult to bring forward arguments of a conclusive kind in favour of either of these processes. The fact that delaminate and invaginate gastrulæ are in several instances found coexisting in the same group renders it certain that there are not two independent phyla of the Metazoa, derived respectively from an invaginate and a delaminate gastrula[119].
[119] It is not difficult to picture a possible derivation of delamination from invagination; while a comparison of the formation of the inner layers (mesoblast and hypoblast) in Ascetta (amongst the Sponges), and in the Echinodermata, shews a very simple way in which it is possible to conceive of a passage of delamination into invagination. In Ascetta the cells, which give rise to the mesoblast and hypoblast, are budded off from the inner wall of the blastosphere, especially at one point; while in Echinodermata (fig. 199) there is a small invaginated sack which gives rise to the hypoblast, while from the walls of this sack amoeboid cells are budded off which give rise to a large part of the mesoblast. If we suppose the hypoblast cells budded off at one point in Ascetta gradually to form an invaginated sack, while the mesoblast cells continued to be budded off as before, we should pass from the delaminate type of Ascetta to the invaginate type of an Echinoderm.
The four most important cases in which the two processes coexist are the Porifera, the Coelenterata, the Nemertea, and the Brachiopoda. In the cases of the Porifera and Coelenterata, there do not appear to me to be any means of deciding which of these processes is derived from the other; but in the Nemertea and the Brachiopoda the case is different. In all the types of Nemertea in which the development is relatively not abbreviated there is an invaginate gastrula, while in the types with a greatly abbreviated development there is a delaminate gastrula. It would seem to follow from this that a delaminate gastrula has here been a secondary result of an abbreviation in the development. In the Brachiopoda, again, the majority of types develop by a process of invagination, while Thecidium appears to develop by delamination; here also the delaminate type would appear to be secondarily derived from the invaginate.
If these considerations are justified, delamination must be in some instances secondarily derived from invagination; and this fact is so far an argument in favour of the more primitive nature of invagination; though it by no means follows that in the invaginate process the steps by which the Metazoa were derived from the Protozoa are preserved.
It does not, therefore, seem possible to decide conclusively in favour of either of these processes by a comparison of the cases where they occur in the same groups.
The relative frequency of the two processes supplies us with another possible means for deciding between them; and there is no doubt that here again the scale inclines towards invagination. It must, however, be borne in mind that the frequency of the process of invagination admits of another possible explanation. There is a continual tendency for the processes of development to be abbreviated and simplified, and it is quite possible that the frequent occurrence of invagination is due to the fact of its being, in most cases, the simplest means by which the two-layered condition can be reached. But this argument can have but little weight until it can be shewn in each case that invagination is a simpler process than delamination; and it is rendered improbable by the cases already mentioned in which delamination has been secondarily derived from invagination.
If it were the case that the blastopore had in all types the same relation to the adult mouth, there would be strong grounds for regarding the invaginate gastrula as an ancestral form; but the fact that this is by no means so is an argument of great weight in favour of some other explanation of the frequency of invagination.
The force of this consideration can best be displayed by a short summary of the fate of the blastopore in different forms.
The fate of the blastopore is so variable that it is difficult even to classify the cases which have been described.
(1) It becomes the permanent mouth in the following forms[120]: Coelenterata.--Pelagia, Cereanthus. Turbellaria.--Leptoplana (?), Thysanozoon. Nemertea.--Pilidium, larvæ of the type of Desor. Mollusca.--In numerous examples of most Molluscan groups, except the Cephalopoda. Chætopoda.--Most Oligochæta, and probably many Polychæta. Gephyrea.--Phascolosoma, Phoronis. Nematelminthes.--Cucullanus. (2) It closes in the position where the mouth is subsequently formed. Coelenterata.--Ctenophora (?). Mollusca.--In numerous examples of most Molluscan groups, except the Cephalopoda. Crustacea.--Cirripedia (?), some Cladocera (Moina) (?). (3) It becomes the permanent anus. Mollusca.--Paludina. Chætopoda.--Serpula and some other types. Echinodermata.--Almost universally, except amongst the Crinoidea. (4) It closes in the position where the anus is subsequently formed. Echinodermata.--Crinoidea. (5) It closes in a position which does not correspond or is not known to correspond[121] either with the future mouth or anus.--Porifera--Sycandra. Coelenterata--Chrysaora*, Aurelia*. Nemertea*--Some larvæ which develop without a metamorphosis. Rotifera*. Mollusca--Cephalopoda. Polyzoa*. Brachiopoda--Argiope, Terebratula, Terebratulina. Chætopoda--Euaxes. Discophora--Clepsine. Gephyrea--Bonellia*. Chætognatha. Crustacea--Decapoda. Chordata.
[120] The above list is somewhat tentative; and future investigations will probably shew that many of the statements at present current about the position of the blastopore are inaccurate.
[121] The forms in which the position of the blastopore in relation to the mouth or anus is not known are marked with an asterisk.
The forms which have been classed together under the last heading vary considerably in the character of the blastopore. In some cases the fact of its not coinciding either with the mouth or anus appears to be due simply to the presence of a large amount of food-yolk. The cases of the Cephalopoda, of Euaxes, and perhaps of Clepsine and Bonellia, are to be explained in this way: in the case of all these forms, except Bonellia, the blastopore has the form of an elongated slit along the ventral surface. This type of blastopore is characteristic of the Mollusca generally, of the Polyzoa, of the Nematelminthes, and very possibly of the Chætopoda and Discophora. In the Chætognatha (fig. 209 B) the blastopore is situated, so far as can be determined, behind the future anus. In many Decapoda the blastopore is placed behind, but not far from, the anus. In the Chordata it is also placed posteriorly to the anus, and, remarkably enough, remains, in a large number of forms, for some time in connection with the neural tube by a neurenteric canal.
The great variations in the character of the gastrula, indicated in the above summary, go far to shew that if the gastrulæ, as we find them in most types, have any ancestral characters, these characters can only be of the most general kind. This may best be shewn by the consideration of a few striking instances. The blastopore in Mollusca has an elongated slit-like form, extending along the ventral surface from the mouth to the anus. In Echinodermata it is a narrow pore, remaining as the anus. In most Chætopoda it is a pore remaining as the mouth, but in some as the anus. In Chordata it is a posteriorly-placed pore, opening into both the archenteron and the neural canal.
It is clearly out of the question to explain all these differences as having connection with the characters of ancestral forms. Many of them can only be accounted for as secondary adaptations for the convenience of development.
The epibolic gastrula of Mammalia (vide pp. 215 and 291) is a still more striking case of a secondary embryonic process, and is not directly derived from the gastrula of the lower Chordata. It probably originated in connection with the loss of food-yolk which took place on the establishment of a placental nutrition for the foetus. The epibolic gastrula of the Scorpion, of Isopods, and of other Arthropoda, seems also to be a derived gastrula. These instances of secondary gastrulæ are very probably by no means isolated, and should serve as a warning against laying too much stress upon the frequency of the occurrence of invagination. The great influence of the food-yolk upon the early development might be illustrated by numerous examples, especially amongst the Chordata (vide Chapter XI.).
If the descendants of a form with a large amount of food-yolk in its ova were to produce ova with but little food-yolk, the type of formation of the germinal layers which would thereby result would be by no means the same as that of the ancestors of the forms with much food-yolk, but would probably be something very different, as in the case of Mammalia. Yet amongst the countless generations of ancestors of most existing forms, such oscillations in the amount of the food-yolk must have occurred in a large number of instances.
The whole of the above considerations point towards the view that the formation of the hypoblast by invagination, as it occurs in most forms at the present day, can have in many instances no special phylogenetic significance, and that the argument from frequency, in favour of invagination as opposed to delamination, is not of prime importance.
A third possible method of deciding between delamination and invagination is to be found in the consideration as to which of these processes occurs in the most primitive forms. If there were any agreement amongst primitive forms as to the type of their development this argument might have some weight. On the whole, delamination is, no doubt, characteristic of many primitive types, but the not infrequent occurrence of invagination in both the Coelenterata and the Porifera--the two groups which would on all hands be admitted to be amongst the most primitive--deprives this argument of much of the value it might otherwise have.
To sum up--considering the almost indisputable fact that both the processes above dealt with have in many instances had a purely secondary origin, no valid arguments can be produced to shew that either of them reproduces the mode of passage between the Protozoa and the ancestral two-layered Metazoa. These conclusions do not, however, throw any doubt upon the fact that the gastrula, however evolved, was a primitive form of the Metazoa; since this conclusion is founded upon the actual existence of adult gastrula forms independently of their occurrence in development.
[FIG. 205. DIAGRAM SHEWING THE FORMATION OF A GASTRULA BY DELAMINATION. (From Lankester.)
Fig. 1, ovum; fig. 2, stage in segmentation; fig. 3, commencement of delamination after the appearance of a central cavity; fig. 4, delamination completed, mouth forming at M. In figs. 1, 2, and 3, Ec. is ectoplasm, and En. is endoplasm. In fig. 4, Ec. is epiblast, and En. hypoblast. E. and F. food particles.]
Though embryology does not at present furnish us with a definite answer to the question how the Metazoa became developed from the Protozoa, it is nevertheless worth while reviewing some of the processes by which this can be conceived to have occurred.
On purely à priori grounds there is in my opinion more to be said for invagination than for any other view.
On this view we may suppose that the colony of Protozoa in the course of conversion into Metazoa had the form of a blastosphere; and that at one pole of this a depression appeared. The cells lining this depression we may suppose to have been amoeboid, and to have carried on the work of digestion; while the remaining cells were probably ciliated. The digestion may be supposed to have been at first carried on in the interior of the cells, as in the Protozoa; but, as the depression became deeper (in order to increase the area of nutritive cells and to retain the food) a digestive secretion probably became poured out from the cells lining it, and the mode of digestion generally characteristic of the Metazoa was thereby inaugurated. It may be noted that an intracellular protozoon type of digestion persists in the Porifera, and appears also to occur in many Coelenterata, Turbellaria, &c., though in most of these cases both kinds of digestion probably go on simultaneously[122].
[122] J. Parker, "On the Histology of Hydra fusca," Quart. Journ. Micr. Science, vol. XX. 1880; and El. Metschnikoff, "Ueb. die intracelluläre Verdauung bei Coelenteraten," Zoologischer Anzeiger, No. 56, vol. III. 1880 and Lankester, "On the intracellular digestion and endoderm of Limnocodium," Quart. Journ. Micr. Science, vol. XXI. 1881.
Another hypothetical mode of passage, which fits in with delamination, has been put forward by Lankester, and is illustrated by fig. 205. He supposes that at the blastosphere stage the fluid in the centre of the colony acquired special digestive properties; the inner ends of the cells having at this stage somewhat different properties from the outer, and the food being still incepted by the surface of the cells (fig. 205, 3). In a later stage of the process the inner portions of the cells became separated off as the hypoblast; while the food, though still ingested in the form of solid particles by the superficial cells, was carried through the protoplasm into the central digestive cavity. Later (fig. 205, 4), the point where the food entered became localised, and eventually a mouth became formed at this point.
The main objection which can be raised against Lankester's view is that it presupposes a type of delamination which does not occur in nature except in Geryonia.
Metschnikoff has propounded a third view with reference to delamination. He starts as before with a ciliated blastosphere. He next supposes the cells from the walls of this to become budded off into the central cavity, as in Eucope (fig. 202), and to lose their cilia. These cells give rise to an internal parenchyma, which carries on an intracellular digestion. At a later stage a central digestive cavity is supposed to be formed. This view of the passage from the protozoon to the metazoon state, though to my mind improbable in itself, fits in very well with the ontogeny of the lower Hydrozoa.
Another view has been put forward by myself in the chapter on the Porifera[123], to the effect that the amphiblastula larva of Calcispongiæ may be a transitional form between the Protozoa and the Metazoa, composed of a hemisphere of nutritive amoeboid cells, and a hemisphere of ciliated cells. The absence of such a larval form in the Coelenterata and higher Metazoa is opposed, however, to this larva being regarded as a transitional form, except for the Porifera.
[123] Vol. II. p. 149.
It is obvious that so long as there is complete uncertainty as to the value to be attached to the early developmental processes, it is not possible to decide from these processes whether there is only a single metazoon phylum or whether there may not be two or more such phyla. At the same time there appear to be strong arguments for regarding the Porifera as a phylum of the Metazoa derived independently from the Protozoa. This seems to me to be shewn (1) by the striking larval peculiarities of the Porifera; (2) by the early development of the mesoblast in the Porifera, which stands in strong contrast to the absence of this layer in the embryos of most Coelenterata; and above all, (3) by the remarkable characters of the system of digestive channels. A further argument in the same direction is supplied by the fact that the germinal layers of the Sponges very probably do not correspond physiologically to the germinal layers of other types. The embryological evidence is insufficient to decide whether the amphiblastula larva is, as suggested above, to be regarded as the larval ancestor of the Porifera.
Homologies of the germinal layers. The question as to how far there is a complete homology between the two primary germinal layers throughout the Metazoa was the third of the questions proposed to be discussed here.
Since there are some Metazoa with only two germinal layers, and other Metazoa with three, and since, as is shewn in the following section, the third layer or mesoblast can only be regarded as a derivative of one or both the primary layers, it is clear that a complete homology between the two primary germinal layers does not exist.
That there is a general homology appears on the other hand hardly open to doubt.
The primary layers are usually continuous with each other, near one or both (when both are present) the openings of the alimentary tract.
As a rule an oral and anal section of the alimentary tract--the stomodæum and proctodæum--are derived from the epiblast; but the limits of both these sections are so variable, sometimes even in closely allied forms, that it is difficult to avoid the conclusion that there is a border-land between the epiblast and hypoblast, which appears by its development to belong in some forms to the epiblast and in other forms to the hypoblast. If this is not the case it is necessary to admit that there are instances in which a very large portion of the alimentary canal is phylogenetically an epiblastic structure. In some of the Isopods, for example, the stomodæum and proctodæum give rise to almost the whole of the alimentary canal with its appendages, except the liver.
The origin of the Mesoblast. A diploblastic condition of the organism preceded, as we have seen, the triploblastic. The epiblast during the diploblastic condition was, as appears from such forms as Hydra, especially the sensory and protective layer, while the hypoblast was the secretory and assimilating layer; both layers giving rise to muscular elements. It must not, however, be supposed that in the early diploblastic ancestors there was a complete differentiation of function, but there is reason to think that both the primary layers retained an indefinite capacity for developing into any form of tissue[124]. The fact of the triploblastic condition being later than the diploblastic proves in a conclusive way that the mesoblast is a derivative of one or both the primary layers. In the Coelenterata we can study the actual origin from the two primary layers of various forms of tissue which in the higher types are derived from the mesoblast[125]. This fact, as well as general à priori considerations, conclusively prove that the mesoblast did not at first originate as a mass of independent cells between the two primary layers, but that in the first instance it gradually arose as differentiations of the two layers, and that its condition in the embryo as an independent layer of undifferentiated cells is a secondary condition, brought about by the general tendency towards a simplification of development, and a retardation of histological differentiation[126].
[124] The Hertwigs (No. 270) have for instance shewn that nervous structures are developed in the hypoblast in the Actinozoa and other Coelenterata.
[125] There is considerable confusion in the use of the names for the embryonic layers. In some cases various tissues formed by differentiations of the primary layers have been called mesoblast. Schultze, and more recently the Hertwigs, have pointed out the inconvenience of this nomenclature. In the case of the Coelenterata it is difficult to decide in certain instances (e.g. Sympodium) whether the cells which give rise to a particular tissue of the adult are to be regarded as forming a mesoblast, i.e. a middle undifferentiated layer of cells, or whether they arise as already histologically differentiated elements from one of the primary layers. The attempt to distinguish by a special nomenclature the epiblast and hypoblast after and before the separation of the mesoblast, which has been made by Allen Thomson (No. 1), appears incapable of being consistently applied, though it is convenient to distinguish a primary and a secondary hypoblast. A proposal of the Hertwigs to adopt special names for the outer and inner limiting membranes of the adult, and for the interposed mass of organs, appears to me unnecessary.
[126] The causes which give rise to a retardation of histological differentiation will be dealt with in the second part of this chapter which deals with larval characters and larval forms.
The Hertwigs have recently attempted (No. 271) to distinguish two types of differentiation of the mesoblast, viz. (1) a direct differentiation from the primitive epithelial cells; (2) a differentiation from primitively indifferent cells budded off into the gelatinous matter between the two primary layers.
It is quite possible that this distinction may be well founded, but no conclusive evidence of the occurrence of the second process has yet been adduced. The Ctenophora are the type upon which special stress is laid, but the early passage of amoeboid cells into the gelatinous tissue, which subsequently become muscular, is very probably an embryonic abbreviation; and it is quite possible that these cells may phylogenetically have originated from epithelial cells provided with contractile processes passing through the gelatinous tissue.
The conversion of non-embryonic connective-tissue cells into muscle cells in the higher types has been described, but very much more evidence is required before it can be accepted as a common occurrence.
In addition to the probably degraded Dicyemidæ and Orthonectidæ, the Coelenterata are the only group in which a true mesoblast is not always present. In other words, the Coelenterata are the only group in which there is not found in the embryo an undifferentiated group of cells from which the majority of the organs situated between the epidermis and the alimentary epithelium are developed.
The organs invariably derived, in the triploblastic forms, from the mesoblast, are the vascular and lymphatic systems, the muscular system, and the greater part of the connective tissue and the excretory and generative (?) systems. On the other hand, the nervous systems (with a few possible exceptions) and organs of sense, the epithelium of most glands, and a few exceptional connective-tissue organs, as for example the notochord, are developed from the two primary layers.
The fact of the first-named set of organs being invariably derived from the mesoblast points to the establishment of the two following propositions:--(1) That with the differentiation of the mesoblast as a distinct layer by the process already explained, the two primary layers lost for the most part the capacity they primitively possessed of giving rise to muscular and connective-tissue differentiations[127], to the epithelium of the excretory organs, and to generative cells. (2) That the mesoblast throughout the triploblastic Metazoa, in so far as these forms have sprung from a common triploblastic ancestor, is an homologous structure.
[127] The connective-tissue test of the Tunicata, though derived from the epiblast, is not really an example of such a differentiation.
The second proposition follows from the first. The mesoblast can only have ceased to be homologous throughout the triploblastica by additions from the two primary layers, and the existence of such additions is negatived by the first proposition.
These two propositions, which hang together, are possibly only approximately true, since it is quite possible that future investigations may shew that differentiations of the two primary layers are not so rare as has been hitherto imagined.
Ranvier[128] finds that the muscles of the sweat-glands are developed from the inner part of the layer of epiblast cells, invaginated to form these glands.
[128] M. L. Ranvier. "Sur la structure des glandes sudoripares." Comptes Rendus, Dec. 29, 1879.
Götte[129] describes the epiblast cells of the larva of Comatula as being at a certain stage contractile and compares them with the epithelio-muscular cells of Hydra. These cells would appear subsequently to be converted into a simple cuticular structure.
[129] A. Götte, "Vergleich. Entwick. d. Comatula mediterranea." Archiv f. mikr. Anat. vol. XII. p. 597.
It is moreover quite possible that fresh differentiations from the two primary layers may have arisen after the triploblastic condition had been established, and by the process of simplification of development and precocious segregation, as Lankester calls it, have become indistinguishable from the normal mesoblast. In spite of these exceptions it is probable that the major part of the muscular system of all existing triploblastic forms has been differentiated from the muscular system of the ancestor or ancestors (if there is more than one phylum) of the triploblastica. In the case of other tissues there are a few instances which might be regarded as examples of an organ primitively developed in one of the two primary layers having become secondarily carried into the mesoblast. The notochord has sometimes been cited as such an organ, but, as indicated in a previous chapter, it is probable that its hypoblastic origin can always be demonstrated.
[FIG. 206. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)
A. Stage when the four hypoblast cells are nearly enclosed. B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.
ep. epiblast; me. mesoblast; bl. blastopore.]
The nervous system, although imbedded in mesoblastic derivates in the adults of all the higher triploblastica, retains with marvellous constancy its epiblastic origin (though it is usually separated from the epiblast prior to its histogenic differentiation); yet in the Cephalopoda, and some other Mollusca, the evidence is in favour of its developing in the mesoblast. Should future investigations confirm these conclusions, a good example will be afforded of an organ changing the layer from which it usually develops[130]. The explanation of such a change would be precisely the same as that already given for the mesoblast as a whole.
[130] The Hertwigs hold that there is a distinct part of the nervous system which was at first differentiated in the mesoblast in many types, amongst others the Mollusca. The evidence in favour of this view is extremely scanty and the view itself appears to me highly improbable.
The actual mode of origin of various tissues, which in the true triploblastic forms arise in mesoblast, can be traced in the Coelenterata[131]. In this group the epiblast and hypoblast both give rise to muscular and connective-tissue elements; and although the main part of the nervous system is formed in the epiblast, it seems certain that in some types nerves may be derived from the hypoblast[132]. These facts are extremely interesting, but it is by no means certain that any conclusions can be directly drawn from them as to the actual origin of the mesoblast in the triploblastic forms, till we know from what diploblastic forms the triploblastica originated. All that they shew is that any of the constituents of the mesoblast may have originated from either of the primitive layers.
[131] The reader is referred for this subject to the valuable memoirs which have been recently published by the Hertwigs, especially to No. 270. He will find a general account of the subject written before the appearance of the Hertwigs' memoir in pp. 180-182 of Volume II. of this treatise.
[132] It would be interesting to know the history of the various nervous structures found in the walls of the alimentary tract in the higher forms. I have shewn (Development of Elasmobranch Fishes, p. 172) that the central part of the sympathetic system is derived from the epiblast. It would however be well to work over the development of Auerbach's plexus.
[FIG. 207. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS PICEUS. (After Kowalevsky.)
A. Section through an embryo at the point where the two germinal folds most approximate. B. Section through an embryo, in the anterior region where the folds of the amnion have not united.
gg. germinal groove; me. mesoblast; am. amnion; yk. yolk.]
For further light as to the origin of the mesoblast, it is necessary to turn to its actual development.
The following summary illustrates the more important modes in which the mesoblast originates.
[FIG. 208. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS. (From Parker; after Reichenbach.)
A. Section through part of the ovum during segmentation. n. nuclei; w.y. white yolk; y.p. yolk pyramids; c. central yolk mass. B. and C. Longitudinal sections of the gastrula stage. a. en. archenteron; b. blastopore; ms. mesoblast; ec. epiblast; hypoblast, distinguished from epiblast by shading. D. Highly magnified view of anterior lip of blastopore, to shew the origin of the primary mesoblast from the wall of the archenteron. p.ms. primary mesoblast; ec. epiblast; en. hypoblast. E. Two hypoblast cells to shew the amoeba-like absorption of yolk spheres. y. yolk; n. nucleus; p. pseudopodial process. F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.); n. nucleus.]
1. It grows inwards from the lips of the blastopore as a pair of bands. In these cases it may originate (a) from cells which are clearly hypoblastic, (b) from cells which are clearly epiblastic, (c) from cells which cannot be regarded as belonging to either layer.
Mollusca.--Gasteropoda, Cephalopoda, and Lamellibranchiata. In Gasteropoda and Lamellibranchiata the mesoblast sometimes originates from a pair of cells at the lips of the blastopore, though very probably some of the elements subsequently come from the epiblast; and in Cephalopoda it begins as a ring of cells round the edge of the blastoderm.
Polyzoa Entoprocta.--It originates from a pair of cells at the lips of the blastopore.
Chætopoda.--Euaxes. It arises as a ridge of cells at the lips of the blastopore (fig. 200).
Gephyrea.--Bonellia. It arises (fig. 206) as an infolding of the epiblastic lips of the blastopore.
Nematelminthes.--Cucullanus. It grows backwards from the hypoblast cells at the persistent oral opening of the blastopore.
Tracheata.--Insecta. It grows inwards from the lips of the germinal groove (fig. 207), which probably represent the remains of a blastopore. Part of the mesoblast is probably also derived from the yolk-cells. A similar though more modified development of the mesoblast occurs in the Araneina (fig. 214).
Crustacea.--Decapoda. It partly grows in from the hypoblastic lips of the blastopore, and is partly derived from the yolk-cells (fig. 208).
[FIG. 209. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A. and C. after Bütschli, and B. after Kowalevsky.)
The three embryos are represented in the same positions. A. Represents the gastrula stage. B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three. C. Represents a later stage, in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore has become closed.
m. mouth; al. alimentary canal; ae. archenteron; bl.p. blastopore; pv. perivisceral cavity; sp. splanchnic mesoblast; so. somatic mesoblast; ge. generative organs.]
2. The mesoblast is developed from the walls of hollow outgrowths of the archenteron, the cavities of which become the body cavity.
Brachiopoda.--The walls of a pair of outgrowths form the whole of the mesoblast.
Chætognatha.--The mesoblast arises in the same manner as in the Brachiopoda (fig. 209).
Echinodermata.--The lining of the peritoneal cavity is developed from the walls of outgrowths of the archenteron, but the greater part of the mesoblast is derived from the amoeboid cells budded off from the walls of the archenteron (fig. 210).
[FIG. 210. LONGITUDINAL SECTION THROUGH AN EMBRYO OF CUCUMARIA DOLIOLUM AT THE END OF THE FOURTH DAY.
Vpv. vaso-peritoneal vesicle; ME. mesenteron; Blp., Ptd. blastopore, proctodæum.]
Enteropneusta (Balanoglossus).--The body cavity is derived from two pairs of alimentary diverticula, the walls of which give rise to the greater part of the mesoblast.
Chordata.--Paired archenteric outgrowths give rise to the whole mesoblast in Amphioxus (fig. 211), and the mode of formation of the mesoblast in other Chordata is probably secondarily derived from this.
3. The cells which will form the mesoblast become marked out very early, and cannot be regarded as definitely springing from either of the primary layers.
Turbellaria.--Leptoplana (fig. 212), Planaria polychroa (?).
Chætopoda.--Lumbricus, &c.
Discophora.
It is very possible that the cases quoted under this head ought more properly to belong to group 1.
4. The mesoblast cells are split off from the epiblast.
Nemertea.--Larva of Desor. The mesoblast is stated to be split off from the four invaginated discs.
5. The mesoblast is split off from the hypoblast.
Nemertea.--Some of the types without a metamorphosis.
Mollusca.--Scaphopoda. It is derived from the lateral and ventral cells of the hypoblast.
Gephyrea.--Phascolosoma.
Vertebrata.--In most of the Ichthyopsida the mesoblast is derived from the hypoblast (fig. 213). In some types (i.e. most of the Amniota) the mesoblast might be described as originating at the lips of the blastopore (primitive streak).
6. The mesoblast is derived from both germinal layers.
Tracheata.--Araneina (fig. 214). It is derived partly from cells split off from the epiblast and partly from the yolk-cells; but it is probable that the statement that the mesoblast is derived from both the germinal layers is only formally accurate; and that the derivation of part of the mesoblast from the yolk-cells is not to be interpreted as a derivation from the hypoblast.
Amniota.--The derivation of the mesoblast of the Amniota from both the primary germinal layers is without doubt a secondary process.
The conclusions to be drawn from the above summary are by no means such as might have been anticipated. The analogy of the Coelenterata would lead us to expect that the mesoblast would be derived partly from the epiblast and partly from the hypoblast. Such, however, is not for the most part the case, though more complete investigations may shew that there are a greater number of instances in which the mesoblast has a mixed origin than might be supposed from the above summary.
[FIG. 211. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.)
A. Section at gastrula stage. B. Section of a somewhat older embryo. C. Section through the anterior part of still older embryo.
np. neural plate; nc. neural canal; mes. archenteron in A, and mesenteron in B and C; ch. notochord; so. mesoblastic somite.]
I have attempted to reduce the types of development of the mesoblast to six; but owing to the nature of the case it is not always easy to distinguish the first of these from the last four. Of the six types the second will on most hands be admitted to be the most remarkable. The formation of hollow outgrowths of the archenteron, the cavities of which give rise to the body cavity, can only be explained on the supposition that the body cavity of the types in which such outgrowths occur is derived from diverticula cut off from the alimentary tract. The lining epithelium of the diverticula--the peritoneal epithelium--is clearly part of the primitive hypoblast, and this part of the mesoblast is clearly hypoblastic in origin.
[FIG. 212. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN THREE STAGES OF DEVELOPMENT. (After Hallez.)
ep. epiblast; m. mesoblast; hy. yolk-cells (hypoblast); bl. blastopore.]
In the case of the Chætognatha (Sagitta), Brachiopoda, and Amphioxus, the whole of the mesoblast originates from the walls of the diverticula; while in the Echinodermata the walls of the diverticula only give rise to the vaso-peritoneal epithelium, the remainder of the mesoblast being derived from amoeboid cells which spring from the walls of the archenteron before the origin of the vaso-peritoneal outgrowths (figs. 199 and 210).
Reserving for the moment the question as to what conclusions can be deduced from the above facts as to the origin of the mesoblast, it is important to determine how far the facts of embryology warrant us in supposing that in the whole of the triploblastic forms the body cavity originated from the alimentary diverticula. There can be but little doubt that the mode of origin of the mesoblast in many Vertebrata, as two solid plates split off from the hypoblast, in which a cavity is secondarily developed, is an abbreviation of the process observable in Amphioxus; but this process approaches in some forms of Vertebrata to the ingrowth of the mesoblast from the lips of the blastopore.
It is, therefore, highly probable that the paired ingrowths of the mesoblast from the lips of the blastopore may have been in the first instance derived from a pair of archenteric diverticula. This process of formation of the mesoblast is, as may be seen by reference to the summary, the most frequent, including as it does the Chætopoda, the Mollusca, the Arthropoda, &c.[133]
[133] The wide occurrence of this process was first pointed out by Rabl. He holds, however, a peculiar modification of the gastræa theory, for which I must refer the reader to his paper (No. 284); according to this theory the mesoblast has sprung from a zone of cells of the blastosphere, at the junction between the cells which will be invaginated and the epiblast cells. In the bilateral blastosphere, from which he holds that all the higher forms (Bilateralia) have originated, these cells had a bilateral arrangement, and thus the bilateral origin of the mesoblast is explained. The origin of the mesoblast from the lips of the blastopore is explained by the position of its mother-cells in the blastosphere. It need scarcely be said that the views already put forward as to the probable mode of origin of the mesoblast, founded on the analogy of the Coelenterata, are quite incompatible with Rabl's theories.
[FIG. 213. TWO SECTIONS OF A YOUNG ELASMOBRANCH EMBRYO, TO SHEW THE MESOBLAST SPLIT OFF AS TWO LATERAL MASSES FROM THE HYPOBLAST.
mg. medullary groove; ep. epiblast; m. mesoblast; hy. hypoblast; n.al. cells formed around the nuclei of the yolk which have entered the hypoblast.]
While there is no difficulty in the view that the body cavity may have originated from a pair of enteric diverticula in the case of the forms where a body cavity is present, there is a considerable difficulty in holding this view, for forms in which there is no body cavity distinct from the alimentary diverticula.
Of these types the Platyelminthes are the most striking. It is, no doubt, possible that a body cavity may have existed in the Platyelminthes, and become lost; and the case of the Discophora, which in their muscular and connective tissue systems as well as in the absence of a body cavity resemble the Platyelminthes, may be cited in favour of this view, in that, being closely related to the Chætopoda, they are almost certainly descended from ancestors with a true body cavity. The usual view of the primitive character of the Platyelminthes, which has much to support it, is, however, opposed to the idea that the body cavity has disappeared.
If Kowalevsky[134] is right in stating that he has found a form intermediate between the Coelenterata and the Platyelminthes, there will be strong grounds for holding that the Platyelminthes are, like the Coelenterata, forms the ancestors of which were not provided with a body cavity.
[134] Zoologischer Anzeiger, No. 52, p. 140. This form has been named by Kowalevsky Coeloplana Metschnikowii. Kowalevsky's description appears, however, to be quite compatible with the view that this form is a creeping Ctenophor, in no way related to the Turbellarians.
Perhaps the triploblastica are composed of two groups, viz. (1) a more ancestral group (the Platyelminthes), in which there is no body cavity as distinct from the alimentary, and (2) a group descended from these, in which two of the alimentary diverticula have become separated from the alimentary tract to form a body cavity (remaining triploblastica). However this may be, the above considerations are sufficient to shew how much there is that is still obscure with reference even to the body cavity.
[FIG. 214. SECTION THROUGH AN EMBRYO OF AGELENA LABYRINTHICA.
The section is represented with the ventral plate upwards. In the ventral plate is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.
yk. yolk divided into large polygonal cells, in several of which are nuclei.]
If embryology gives no certain sound as to the questions just raised with reference to the body cavity, still less is it to be hoped that the remaining questions with reference to the origin of the mesoblast can be satisfactorily answered. It is clear, in the first place, from an inspection of the summary given above, that the process of development of the mesoblast is, in all the higher forms, very much abbreviated and modified. Not only is its differentiation relatively deferred, but it does not in most cases originate, as it must have done to start with, as a more or less continuous sheet, split off from parts of one or both the primary layers. It originates in most cases from the hypoblast, and although the considerations already urged preclude us from laying very great stress on this mode of origin, yet the derivation of the mesoblast from the walls of archenteric outgrowths suggests the view that the whole, or at any rate the greater part, of the mesoblast primitively arose by a process of histogenic differentiation from the walls of the archenteron or rather from diverticula of these walls. This view, which was originally put forward by myself (No. 260), appears at first sight very improbable, but if the statement of the Hertwigs (No. 270), that there is a large development of a hypoblastic muscular system in the Actinozoa, is well founded, it cannot be rejected as impossible. Lankester (No. 279), on the other hand, has urged that the mode of origin of the mesoblast in the Echinodermata is more primitive; and that the amoeboid cells which here give rise to the muscular and connective tissues represent cells which originally arose from the whole inner surface of the epiblast. It is, however, to be noted that even in the Echinodermata the amoeboid cells actually arise from the hypoblast, and their mode of origin may, therefore, be used to support the view that the main part of the muscular system of higher types is derived from the primitive hypoblast.
The great changes which have taken place in the development of the mesoblast would be more intelligible on this view than on the view that the major part of the mesoblast primitively originated from the epiblast. The presence of food-yolk is much more frequent in the hypoblast than in the epiblast; and it is well known that a large number of the changes in early development are caused by food-yolk. If, therefore, the mesoblast has been derived from the hypoblast, many more changes might be expected to have been introduced into its early development than if it had been derived from the epiblast. At the same time the hypoblastic origin of the mesoblast would assist in explaining how it has come about that the development of the nervous system is almost always much less modified than that of the mesoblast, and that the nervous system is not, as might, on the grounds of analogy, have been anticipated, as a rule secondarily developed in the mesoblast.
The Hertwigs have recently suggested in their very interesting memoir (No. 271) that the Triploblastica are to be divided into two phyla, (1) the Enterocoela, and (2) the Pseudocoela; the former group containing the Chætopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda, Echinodermata, Enteropneusta and Chordata; and the latter the Mollusca, Polyzoa, the Rotifera, and Platyelminthes.
The Enterocoela are forms in which the primitive alimentary diverticula have given origin to the body cavity, while the major part of the muscular system has originated from the epithelial walls of these diverticula, part however being in many cases also derived from the amoeboid cells, called by them mesenchyme, by the second process of mesoblastic differentiation mentioned on p. 347.
In the Pseudocoela the muscular system has become differentiated from mesenchyme cells; while the body cavity, where it exists, is merely a split in the mesenchyme.
It is impossible for me to attempt in this place to state fully, or do justice to, the original and suggestive views contained in this paper. The general conclusion I cannot however accept. The views of the Hertwigs depend to a large extent upon the supposition that it is possible to distinguish histologically muscle cells derived from epithelial cells, from those derived from mesenchyme cells. That in many cases, and strikingly so in the Chordata, the muscle cells retain clear indications of their primitive origin from epithelial cells, I freely admit; but I do not believe either that its histological character can ever be conclusive as to the non-epithelial origin of a muscle cell, or that its derivation in the embryo from an indifferent amoeboid cell is any proof that it did not, to start with, originate from an epithelial cell.
I hold, as is clear from the preceding statements, that such immense secondary modifications have taken place in the development of the mesoblast, that no such definite conclusions can be deduced from its mode of development as the Hertwigs suppose.
In support of the view that the early character of embryonic cells is no safe index as to their phylogenetic origin, I would point to the few following facts.
(1) In the Porifera and many of the Coelenterata (Eucope polystyla, Geryonia, &c.) the hypoblast (endoderm) originates from cells, which according to the Hertwigs' views ought to be classed as mesenchyme.
(2) In numerous instances muscles which have, phylogenetically, an undoubted epithelial origin, are ontogenetically derived from cells which ought to be classed as mesenchyme. The muscles of the head in all the higher Vertebrata, in which the head cavities have disappeared, are examples of this kind; the muscles of many of the Tracheata, notably the Araneina, must also be placed in the same category.
(3) The Mollusca are considered by the Hertwigs to be typical Pseudocoela. A critical examination of the early development of the mesoblast in these forms demonstrates however that with reference to the mesoblast they must be classed in the same group as the Chætopoda. The mesoblast (Vol. II. p. 227) clearly originates as two bands of cells which grow inwards from the blastopore, and in some forms (Paludina, Vol. II. fig. 107) become divided into a splanchnic and somatic layer, with a body cavity between them. All these processes are such as are, in other instances, admitted to indicate Enterocoelous affinities.
The subsequent conversion of the mesoblast elements into amoeboid cells, out of which branched muscles are formed, is in my opinion simply due to the envelopment of the soft Molluscan body within a hard shell.
In addition to these instances I may point out that the distinction between the Pseudocoela and Enterocoela utterly breaks down in the case of the Discophora, and the Hertwigs have made no serious attempt to discuss the characters of this group in the light of their theory, and that the derivation of the Echinoderm muscles from mesenchyme cells is a difficulty which is very slightly treated.
II. LARVAL FORMS: THEIR NATURE, ORIGIN AND AFFINITIES.
Preliminary considerations. In a general way two types of development may be distinguished, viz. a foetal type and a larval type. In the foetal type animals undergo the whole or nearly the whole of their development within the egg or within the body of the parent, and are hatched in a condition closely resembling the adult; and in the larval type they are born at an earlier stage of development, in a condition differing to a greater or less extent from the adult, and reach the adult state either by a series of small steps, or by a more or less considerable metamorphosis.
The satisfactory application of embryological data to morphology depends upon a knowledge of the extent to which the record of ancestral history has been preserved in development. Unless secondary changes intervened this record would be complete; it becomes therefore of the first importance to the embryologist to study the nature and extent of the secondary changes likely to occur in the foetal or the larval state.
The principles which govern the perpetuation of variations which occur in either the larval or the foetal state are the same as those for the adult condition. Variations favourable to the survival of the species are equally likely to be perpetuated, at whatever period of life they occur, prior to the loss of the reproductive powers. The possible nature and extent of the secondary changes which may have occurred in the developmental history of forms, which have either a long larval existence, or which are born in a nearly complete condition, is primarily determined by the nature of the favourable variations which can occur in each case.
Where the development is a foetal one, the favourable variations which can most easily occur are--(1) abbreviations, (2) an increase in the amount of food-yolk stored up for the use of the developing embryo. Abbreviations take place because direct development is always simpler, and therefore more advantageous; and, owing to the fact of the foetus not being required to lead an independent existence till birth, and of its being in the meantime nourished by food-yolk, or directly by the parent, there are no physiological causes to prevent the characters of any stage of the development, which are of functional importance during a free but not during a foetal existence, from disappearing from the developmental history. All organs of locomotion and nutrition not required by the adult will, for this reason, obviously have a tendency to disappear or to be reduced in foetal developments; and a little consideration will shew that the ancestral stages in the development of the nervous and muscular systems, organs of sense, and digestive system will be liable to drop out or be modified, when a simplification can thereby be effected. The circulatory and excretory systems will not be modified to the same extent, because both of them are usually functional during foetal life.
The mechanical effects of food-yolk are very considerable, and numerous instances of its influence will be found in the earlier chapters of this work[135]. It mainly affects the early stages of development, i.e. the form of the gastrula, &c.
[135] For numerous instances of this kind, vide Chapter XI. of Vol. III.
The favourable variations which may occur in the free larva are much less limited than those which can occur in the foetus. Secondary characters are therefore very numerous in larvæ, and there may even be larvæ with secondary characters only, as, for instance, the larvæ of Insects.
In spite of the liability of larvæ to acquire secondary characters, there is a powerful counterbalancing influence tending towards the preservation of ancestral characters, in that larvæ are necessarily compelled at all stages of their growth to retain in a functional state such systems of organs, at any rate, as are essential for a free and independent existence. It thus comes about that, in spite of the many causes tending to produce secondary changes in larvæ, there is always a better chance of larvæ repeating, in an unabbreviated form, their ancestral history, than is the case with embryos, which undergo their development within the egg.
It may be further noted as a fact which favours the relative retention by larvæ of ancestral characters, that a secondary larval stage is less likely to be repeated in development than an ancestral stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain of development, to drop out by the occurrence of a reversion to the original type of development.
The relative chances of the ancestral history being preserved in the foetus or the larva may be summed up in the following way:--There is a greater chance of the ancestral history being lost in forms which develop in the egg; and of its being masked in those which are hatched as larvæ.
The evidence from existing forms undoubtedly confirms the a priori considerations just urged[136]. This is well shewn by a study of the development of Echinodermata, Nemertea, Mollusca, Crustacea, and Tunicata. The free larvæ of the four first groups are more similar amongst themselves than the embryos which develop directly, and since this similarity cannot be supposed to be due to the larvæ having been modified by living under precisely similar conditions, it must be due to their retaining common ancestral characters. In the case of the Tunicata the free larvæ retain much more completely than the embryos certain characters such as the notochord, the cerebrospinal canal, etc., which are known to be ancestral.
[136] It has long been known that land and freshwater forms develop without a metamorphosis much more frequently than marine forms. This is probably to be explained by the fact that there is not the same possibility of a land or freshwater species extending itself over a wide area by the agency of free larvæ, and there is, therefore, much less advantage in the existence of such larvæ; while the fact of such larvæ being more liable to be preyed upon than eggs, which are either concealed, or carried about by the parent, might render a larval stage absolutely disadvantageous.
Types of Larvæ.--Although there is no reason to suppose that all larval forms are ancestral, yet it seems reasonable to anticipate that a certain number of the known types of larvæ would retain the characters of the ancestors of the more important phyla of the animal kingdom.
Before examining in detail the claims of various larvæ to such a character, it is necessary to consider somewhat more at length the kind of variations which are most likely to occur in larval forms.
It is probable a priori that there are two kinds of larvæ, which may be distinguished as primary and secondary larvæ. Primary larvæ are more or less modified ancestral forms, which have continued uninterruptedly to develop as free larvæ from the time when they constituted the adult form of the species. Secondary larvæ are those which have become introduced into the ontogeny of species, the young of which were originally hatched with all the characters of the adult; such secondary larvæ may have originated from a diminution of food-yolk in the egg and a consequently earlier commencement of a free existence, or from a simple adaptive modification in the just hatched young. Secondary larval forms may resemble the primary larval forms in cases where the ancestral characters were retained by the embryo in its development within the egg; but in other instances their characters are probably entirely adaptive.
Causes tending to produce secondary changes in larvæ.--The modes of action of natural selection on larvæ may probably be divided more or less artificially into two classes.
1. The changes in development directly produced by the existence of a larval stage. 2. The adaptive changes in a larva acquired in the ordinary course of the struggle for existence.
The changes which come under the first head consist essentially in a displacement in the order of development of certain organs. There is always a tendency in development to throw back the differentiation of the embryonic cells into definite tissues to as late a date as possible. This takes place in order to enable the changes of form, which every organ undergoes, in repeating even in an abbreviated way its phylogenetic history, to be effected with the least expenditure of energy. Owing to this tendency it comes about that when an organism is hatched as a larva many of the organs are still in an undifferentiated state, although the ancestral form which this larva represents had all its organs fully differentiated. In order, however, that the larva may be enabled to exist as an independent organism, certain sets of organs, e.g. the muscular, nervous, and digestive systems, have to be histologically differentiated. If the period of foetal life is shortened, an earlier differentiation of certain organs is a necessary consequence; and in almost all cases the existence of a larval stage causes a displacement in order of development of organs, the complete differentiation of many organs being retarded relatively to the muscular, nervous, and digestive systems.
The possible changes under the second head appear to be unlimited. There is, so far as I see, no possible reason why an indefinite number of organs should not be developed in larvæ to protect them from their enemies, and to enable them to compete with larvæ of other species, and so on. The only limit to such development appears to be the shortness of larval life, which is not likely to be prolonged, since, ceteris paribus, the more quickly maturity is reached the better it is for the species.
A very superficial examination of marine larvæ shews that there are certain peculiarities common to most of them, and it is important to determine how far such peculiarities are to be regarded as adaptive. Almost all marine larvæ are provided with well-developed organs of locomotion, and transparent bodies. These two features are precisely those which it is most essential for such larvæ to have. Organs of locomotion are important, in order that larvæ may be scattered as widely as possible, and so disseminate the species; and transparency is very important in rendering larvæ invisible, and so less liable to be preyed upon by their numerous enemies[137].
[137] The phosphorescence of many larvæ is very peculiar. I should have anticipated that phosphorescence would have rendered them much more liable to be captured by the forms which feed upon them; and it is difficult to see of what advantage it can be to them.
These considerations, coupled with the fact that almost all free-swimming animals, which have not other special means of protection, are transparent, seem to shew that the transparency of larvæ at all events is adaptive; and it is probable that organs of locomotion are in many cases specially developed, and not ancestral.
Various spinous processes on the larvæ of Crustacea and Teleostei are also examples of secondarily acquired protective organs.
These general considerations are sufficient to form a basis for the discussion of the characters of the known types of larvæ.
The following table contains a list of the more important of such larval forms:
DICYEMIDÆ.--The Infusoriform larva (vol. II. fig. 62). PORIFERA.--(a) The Amphiblastula larva (fig. 215), with one-half of the body ciliated, and the other half without cilia; (b) an oval uniformly ciliated larva, which may be either solid or have the form of a vesicle. COELENERATA.--The planula (fig. 216). TURBELLARIA.--(a) The eight-lobed larva of Müller (fig. 222); (b) the larva of Götte and Metschnikoff, with some Pilidium characters. NEMERTEA.--The Pilidium (fig. 221). TREMATODA.--The Cercaria. ROTIFERA.--The Trochosphere-like larvæ of Brachionus (fig. 217) and Lacinularia. MOLLUSCA.--The Trochosphere larva (fig. 218), and the subsequent Veliger larva (fig. 219). BRACHIOPODA.--The three-lobed larva, with a postoral ring of cilia (fig. 220). POLYZOA.--A larval form with a single ciliated ring surrounding the mouth, and an aboral ciliated ring or disc (fig. 228). CHÆTOPODA.--Various larval forms with many characters like those of the molluscan Trochosphere, frequently with distinct transverse bands of cilia. They are classified as Atrochæ, Mesotrochæ, Telotrochæ (fig. 225 A and fig. 226), Polytrochæ, and Monotrochæ (fig. 225 B). GEPHYREA NUDA.--Larval forms like those of preceding groups. A specially characteristic larva is that of Echiurus (fig. 227). GEPHYREA TUBICOLA.--Actinotrocha (fig. 230), with a postoral ciliated ring of arms. MYRIAPODA.--A functionally hexapodous larval form is common to all the Chilognatha (vol. II. fig. 174). INSECTA.--Various secondary larval forms. CRUSTACEA.--The Nauplius (vol. II. fig. 208) and the Zoæa (vol. II. fig. 210). ECHINODERMATA.--The Auricularia (fig. 223 A), the Bipinnaria (fig. 223 B), and the Pluteus (fig. 224), and the transversely-ringed larvæ of Crinoidea (vol. II. fig. 268). The three first of which can be reduced to a common type (fig. 231 C). ENTEROPNEUSTA.--Tornaria (fig. 229). UROCHORDA (TUNICATA).--The tadpole-like larva (vol. III. fig. 8). GANOIDEI.--A larva with a disc with adhesive papillæ in front of the mouth (vol. III. fig. 67). ANURUS AMPHIBIA.--The tadpole (vol. III. fig. 80).
[FIG. 215. TWO FREE STAGES IN THE DEVELOPMENT OF SYCANDRA RAPHANUS. (After Schultze.)
A. Amphiblastula stage. B. Stage after the ciliated cells have commenced to be invaginated.
c.s. segmentation cavity; ec. granular epiblast cells; en. ciliated hypoblast cells.]
[FIG. 216. THREE LARVAL STAGES OF EUCOPE PLOYSTYLA. (After Kowalevsky.)
A. Blastosphere stage with hypoblast spheres becoming budded into the central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity.
ep. epiblast; hy. hypoblast; al. gastric cavity.]
Of the larval forms included in the above list a certain number are probably without affinities outside the group to which they belong. This is the case with the larvæ of the Myriapoda, the Crustacean larvæ, and with the larval forms of the Chordata. I shall leave these forms out of consideration.
There are, again, some larval forms which may possibly turn out hereafter to be of importance, but from which, in the present state of our knowledge, we cannot draw any conclusions. The infusoriform larva of the Dicyemidæ, and the Cercaria of the Trematodes, are such forms.
Excluding these and certain other forms, we have finally left for consideration the larvæ of the Coelenterata, the Turbellaria, the Rotifera, the Nemertea, the Mollusca, the Polyzoa, the Brachiopoda, the Chætopoda, the Gephyrea, the Echinodermata, and the Enteropneusta.
The larvæ of these forms can be divided into two groups. The one group contains the larva of the Coelenterata or Planula, the other group the larvæ of all the other forms.
The Planula (fig. 216) is characterised by its extreme simplicity. It is a two-layered organism, with a form varying from cylindrical to oval, and usually a radial symmetry. So long as it remains free it is not usually provided with a mouth, and it is as yet uncertain whether or no the absence of a mouth is to be regarded as an ancestral character. The Planula is very probably the ancestral form of the Coelenterata.
[FIG. 217. EMBRYO OF BRACHIONUS URCEOLARIS, SHORTLY BEFORE IT IS HATCHED. (After Salensky.)
m. mouth; ms. masticatory apparatus; me. mesenteron; an. anus; ld. lateral gland; ov. ovary; t. tail (foot); tr. trochal disc; sg. supraoesophageal ganglion.]
The larvæ of almost all the other groups, although they may be subdivided into a series of very distinct types, yet agree in the possession of certain common characters[138]. There is a more or less dome-shaped dorsal surface, and a flattened or concave ventral surface, containing the opening of the mouth, and usually extending posteriorly to the opening of the anus, when such is present.
[138] The larva of the Brachiopoda does not possess most of the characters mentioned below. It is probably, all the same, a highly differentiated larval form belonging to this group.
The dorsal dome is continued in front of the mouth to form a large præoral lobe.
There is usually present at first an uniform covering of cilia; but in the later larval stages there are almost always formed definite bands or rings of long cilia, by which locomotion is effected. These bands are often produced into arm-like processes.
The alimentary canal has, typically, the form of a bent tube with a ventral concavity, constituted (when an anus is present) of three sections, viz. an oesophagus, a stomach, and a rectum. The oesophagus and sometimes the rectum are epiblastic in origin, while the stomach always and the rectum usually are derived from the hypoblast[139].
[139] There is some uncertainty as to the development of the oesophagus in the Echinodermata, but recent researches appear to indicate that it is developed from the hypoblast.
[FIG. 218. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM. (From Lankester.)
f. foot; ot. otocyst; m. mouth; v. velum; ng. nerve ganglion; ry. residual yolk spheres; shs. shell-gland; i. intestine.]
To the above characters may be added a glass-like transparency; and the presence of a widish space possibly filled with gelatinous tissue, and often traversed by contractile cells, between the alimentary tract and the body wall.
Considering the very profound differences which exist between many of these larvæ, it may seem that the characters just enumerated are hardly sufficient to justify my grouping them together. It is, however, to be borne in mind that my grounds for doing so depend quite as much upon the fact that they constitute a series without any great breaks in it, as upon the existence of characters common to the whole of them. It is also worth noting that most of the characters which have been enumerated as common to the whole of these larvæ are not such secondary characters as (in accordance with the considerations used above) might be expected to arise from the fact of their being subjected to nearly similar conditions of life. Their transparency is, no doubt, such a secondary character, and it is not impossible that the existence of ciliated bands may be so also; but it is quite possible that if, as I suppose, these larvæ reproduce the characters of some ancestral form, this form may have existed at a time when all marine animals were free-swimming, and that it may, therefore, have been provided with at least one ciliated band.
[FIG. 219. LARVÆ OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE. (From Gegenbaur.)
A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia). v. velum; c. shell; p. foot; op. operculum; t. tentacle.]
[FIG. 220. LARVA OF ARGIOPE. (From Gegenbaur; after Kowalevsky.)
m. mantle; b. setæ; d. archenteron.]
The detailed consideration of the characters of these larvæ, given below, supports this view.
This great class of larvæ may, as already stated, be divided into a series of minor subdivisions. These subdivisions are the following:
1. The Pilidium Group.--This group is characterised by the mouth being situated nearly in the centre of the ventral surface, and by the absence of an anus. It includes the Pilidium of the Nemertines (fig. 221), and the various larvæ of marine Dendrocoela (fig. 222). At the apex of the præoral lobe a thickening of epiblast may be present, from which (fig. 232) a contractile cord sometimes passes to the oesophagus.
[FIG. 221. TWO STAGES IN THE DEVELOPMENT OF PILIDIUM. (After Metschnikoff.)
ae. archenteron; oe. oesophagus; st. stomach; am. amnion; pr.d. prostomial disc; po.d. metastomial disc; c.s. cephalic sack (lateral pit).]
2. The Echinoderm Group.--This group (figs. 223, 224 and 231 C) is characterised by the presence of a longitudinal postoral band of cilia, by the absence of special sense organs in the præoral region, and by the development of the body cavity as an outgrowth of the alimentary tract. The three typical divisions of the alimentary tract are present, and there is a more or less developed præoral lobe. This group only includes the larvæ of the Echinodermata.
3. The Trochosphere Group.--This group (figs. 225, 226) is characterised by the presence of a præoral ring of long cilia, the region in front of which forms a great part of the præoral lobe. The mouth opens immediately behind the præoral ring of cilia, and there is very often a second ring of short cilia parallel to the main ring, immediately behind the mouth. The function of the ring of short cilia is nutritive, in that its cilia are employed in bringing food to the mouth; while the function of the main ring is locomotive. A perianal patch or ring of cilia is often present (fig. 225 A), and in many forms intermediate rings are developed between the præoral and perianal rings.
[FIG. 222. A. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER HATCHING. VIEWED FROM THE SIDE. (After Hallez.) m. mouth.
B. MÜLLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED FROM THE VENTRAL SURFACE. (After Müller.) The ciliated band is represented by the black line. m. mouth; u.l. upper lip.]
The præoral lobe is usually the seat of a special thickening of epiblast, which gives rise to the supraoesophageal ganglion of the adult. On this lobe optic organs are very often developed in connection with the supraoesophageal ganglion, and a contractile band frequently passes from this region to the oesophagus.
The alimentary tract is formed of the three typical divisions.
The body cavity is not developed directly as an outgrowth of the alimentary tract, though the process by which it originates is very probably secondarily modified from a pair of alimentary outgrowths.
Paired excretory organs, opening to the exterior and into the body cavity, are often present (fig. 226 nph).
This type of larva is found in the Rotifera (fig. 217) (in which it is preserved in the adult state), the Chætopoda (figs. 225 and 226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and the Polyzoa (fig. 228)[140].
[140] For a discussion as to the structure of the Polyzoon larva, vide Vol. II. p. 305.
[FIG. 223. A. THE LARVA OF A HOLOTHUROID.
B. THE LARVA OF AN ASTEROID.
m. mouth; st. stomach; a. anus; l.c. primitive longitudinal ciliated band; pr.c. præoral ciliated band.]
4. Tornaria.--This larva (fig. 229) is intermediate in most of its characters between the larvæ of the Echinodermata (more especially the Bipinnaria) and the Trochosphere. It resembles Echinoderm larvæ in the possession of a longitudinal ciliated band (divided into a præoral and a postoral ring), and in the derivation of the body cavity and water-vascular vesicle from alimentary diverticula; and it resembles the Trochosphere in the presence of sense organs on the præoral lobe, in the existence of a perianal ring of cilia, and in the possession of a contractile band passing from the præoral lobe to the oesophagus.
[FIG. 224. A LARVA OF STRONGYLOCENTRUS. (From Agassiz.)
m. mouth; a. anus; o. oesophagus; d. stomach; c. intestine; ['v]. and v. ciliated ridges; w. water-vascular tube; r. calcareous rods.]
5. Actinotrocha.--The remarkable larva of Phoronis (fig. 230), known as Actinotrocha, is characterised by the presence of (1) a postoral and somewhat longitudinal ciliated ring produced into tentacles, and (2) a perianal ring. It is provided with a præoral lobe, and a terminal or somewhat dorsal anus.
6. The larva of the Brachiopoda articulata (fig. 220).
The relationships of the six types of larval forms thus briefly characterised have been the subject of a considerable amount of controversy, and the following suggestions on their affinities must be viewed as somewhat speculative. The Pilidium type of larva is in some important respects less highly differentiated than the larvæ of the five other groups. It is, in the first place, without an anus; and there are no grounds for supposing that the anus has become lost by retrogressive changes. If for the moment it is granted that the Pilidium larva represents more nearly than the larvæ of the other groups the ancestral type of larva, what characters are we led to assign to the ancestral form which this larva repeats?
[FIG. 225. TWO CHÆTOPOD LARVÆ. (From Gegenbaur.)
o. mouth; i. intestine; a. anus; v. præoral ciliated band; w. perianal ciliated band.]
In the first place, this ancestral form, of which fig. 231 A is an ideal representation, would appear to have had a dome-shaped body, with a flattened oral surface and a rounded aboral surface. Its symmetry was radial, and in the centre of the flattened oral surface was placed the mouth, and round its edge was a ring of cilia. The passage of a Pilidium-like larva into the vermiform bilateral Platyelminth form, and therefore it may be presumed of the ancestral form which this larva repeats, is effected by the larva becoming more elongated, and by the region between the mouth and one end of the body becoming the præoral region, and by an outgrowth between the mouth and the opposite end developing into the trunk, an anus becoming placed at its extremity in the higher forms.
If what has been so far postulated is correct, it is clear that this primitive larval form bears a very close resemblance to a simplified free-swimming Coelenterate (Medusa), and that the conversion of such a radiate form into the bilateral took place, not by the elongation of the aboral surface, and the formation of an anus there, but by the unequal elongation of the oral face, an anterior part, together with the dome above it, forming a præoral lobe, and a posterior outgrowth the trunk (figs. 226 and 233); while the aboral surface became the dorsal surface.
[FIG. 226. POLYGORDIUS LARVA. (After Hatschek.)
m. mouth; sg. supraoesophageal ganglion; nph. nephridion; me.p. mesoblastic band; an. anus; ol. stomach.]
This view fits in very well with the anatomical resemblances between the Coelenterata and the Turbellaria[141], and shews, if true, that the ventral and median position of the mouth in many Turbellaria is the primitive one.
[141] Vide Vol. II. pp. 179 and 191. In this connection attention may be called to Coeloplana Metschnikowii, a form described by Kowalevsky, Zoologischer Anzeiger, No. 52, p. 140, as being intermediate between the Ctenophora and the Turbellaria. As already mentioned, there does not appear to me to be sufficient evidence to prove that this form is not merely a creeping Ctenophor.
[FIG. 227. LARVA OF ECHIURUS. (After Salensky.)
m. mouth; an. anus; sg. supraoesophageal ganglion (?).]
[FIG. 228. DIAGRAM OF A LARVA OF THE POLYZOA.
m. mouth; an. anus; st. stomach; s. ciliated disc.]
The above suggestion as to the mode of passage from the radial into the bilateral form differs largely from that usually held. Lankester[142], for instance, gives the following account of this passage:
[142] Quart. Journ. of Micr. Science, Vol. XVII. pp. 422-3.
"It has been recognised by various writers, but notably by Gegenbaur and Haeckel, that a condition of radiate symmetry must have preceded the condition of bilateral symmetry in animal evolution. The Diblastula may be conceived to have been at first absolutely spherical with spherical symmetry. The establishment of a mouth led necessarily to the establishment of a structural axis passing through the mouth, around which axis the body was arranged with radial symmetry. This condition is more or less perfectly maintained by many Coelenterates, and is reassumed by degradation of higher forms (Echinoderms, some Cirrhipedes, some Tunicates). The next step is the differentiation of an upper and a lower surface in relation to the horizontal position, with mouth placed anteriorly, assumed by the organism in locomotion. With the differentiation of a superior and inferior surface, a right and a left side, complementary one to the other, are necessarily also differentiated. Thus the organism becomes bilaterally symmetrical. The Coelentera are not wanting in indications of this bilateral symmetry, but for all other higher groups of animals it is a fundamental character. Probably the development of a region in front of, and dorsal to the mouth, forming the Prostomium, was accomplished pari passu with the development of bilateral symmetry. In the radially symmetrical Coelentera we find very commonly a series of lobes of the body-wall or tentacles produced equally--with radial symmetry, that is to say--all round the mouth, the mouth terminating the main axis of the body--that is to say, the organism being 'telostomiate.' The later fundamental form, common to all animals above the Coelentera, is attained by shifting what was the main axis of the body--so that it may be described now as the 'enteric' axis; whilst the new main axis, that parallel with the plane of progression, passes through the dorsal region of the body running obliquely in relation to the enteric axis. Only one lobe or outgrowth of those radially disposed in the telostomiate organisms now persists. This lobe lies dorsally to the mouth, and through it runs the new main axis. This lobe is the Prostomium, and all the organisms which thus develop a new main axis, oblique to the old main axis, may be called prostomiate."
It will be seen from this quotation that the aboral part of the body is supposed to elongate to form the trunk, while the præoral region is derived from one of the tentacles.
Before proceeding to further considerations as to the origin of the Bilateralia, suggested by the Pilidium type of larva, it is necessary to enter into a more detailed comparison between our larval forms.
A very superficial consideration of the characters of these forms brings to light two important features in which they differ, viz.:
(1) In the presence or absence of sense organs on the præoral lobe.
[FIG. 229. TWO STAGES IN THE DEVELOPMENT OF TORNARIA. (After Metschnikoff.)
The black lines represent the ciliated bands.
m. mouth; an. anus; br. branchial cleft; ht. heart; c. body cavity between splanchnic and somatic mesoblast layers; w. so-called water-vascular vesicle; v. circular blood-vessel.]
(2) In the presence or absence of outgrowths from the alimentary tract to form the body cavity.
The larvæ of the Echinodermata and Actinotrocha (?) are without sense organs on the præoral lobe, while the other types of larvæ are provided with them. Alimentary diverticula are characteristic of the larvæ of the Echinodermata and of Tornaria.
If the conclusion already arrived at to the effect that the prototype of the six larval groups was descended from a radiate ancestor is correct, it appears to follow that the nervous system, in so far as it was differentiated, had primitively a radiate form; and it is also probably true that there were alimentary diverticula in the form of radial pouches, two of which may have given origin to the paired diverticula which become the body cavity in such types as the Echinodermata, Sagitta, etc. If these two points are granted, the further conclusions seem to follow--(1) that the ganglion and sense organs of the præoral lobe were secondary structures, which arose (perhaps as differentiations of an original circular nerve ring) after the assumption of a bilateral form; and (2) that the absence of these organs in the larvæ of the Echinodermata and Actinotrocha (?) implies that these larvæ retain, so far, more primitive characters than the Pilidium. The same may be said of the alimentary diverticula. There are thus indications that in two important points the Echinoderm larvæ are more primitive than the Pilidium.
[FIG. 230. ACTINOTROCHA. (After Metschnikoff.)
m. mouth; an. anus.]
The above conclusions with reference to the Pilidium and Echinoderm larvæ involve some not inconsiderable difficulties, and suggest certain points for further discussion.
In the first place it is to be noted that the above speculations render it probable that the type of nervous system from which that found in the adults of the Echinodermata, Platyelminthes, Chætopoda, Mollusca, etc., is derived, was a circumoral ring, like that of Medusæ, with which radially arranged sense organs may have been connected; and that in the Echinodermata this form of nervous system has been retained, while in the other types it has been modified. Its anterior part may have given rise to supraoesophageal ganglia and organs of vision; these being developed on the assumption of a bilaterally symmetrical form, and the consequent necessity arising for the sense organs to be situated at the anterior end of the body. If this view is correct, the question presents itself as to how far the posterior part of the nervous system of the Bilateralia can be regarded as derived from the primitive radiate ring.
[FIG. 231. THREE DIAGRAMS REPRESENTING THE IDEAL EVOLUTION OF VARIOUS LARVAL FORMS.
A. Ideal ancestral larval form. B. Larval form from which the Trochosphere larva may have been derived. C. Larval form from which the typical Echinoderm larva may have been derived.
m. mouth; an. anus; st. stomach; s.g. supraoesophageal ganglion. The black lines represent the ciliated bands.]
A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords united in front and behind--exactly such a nervous system, in fact, as is present in many Nemertines[143] (the Enopla and Pelagonemertes), in Peripatus[144], and in primitive molluscan types (Chiton, Fissurella, etc.). From the lateral parts of this ring it would be easy to derive the ventral cord of the Chætopoda and Arthropoda. It is especially deserving of notice in connection with the nervous system of the above-mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on the dorsal side of the intestine. As is at once obvious, by referring to the diagram (fig. 231 B), this is the position this commissure ought, undoubtedly, to occupy if derived from part of a nerve-ring which originally followed more or less closely the ciliated edge of the body of the supposed radiate ancestor.
[143] Vide Hubrecht, "Zur Anat. und Phys. d. Nerven-System. d. Nemertinen," Kön. Akad. Wiss., Amsterdam; and "Researches on the Nervous System of Nemertines," Quart. Journ. of Micr. Science, 1880.
[144] Vide F. M. Balfour, "On some points in the Anat. of Peripatus capensis," Quart. Journ. of Micr. Science, Vol. XIX. 1879.
The fact of this arrangement of the nervous system being found in so primitive a type as the Nemertines tends to establish the views for which I am arguing; the absence or imperfect development of the two longitudinal cords in Turbellarians may very probably be due to the posterior part of the nerve-ring having atrophied in this group.
It is by no means certain that this arrangement of the nervous system in some Mollusca and in Peripatus is primitive, though it may be so.
In the larvæ of the Turbellaria the development of sense organs in the præoral region is very clear (fig. 222 B); but this is by no means so obvious in the case of the true Pilidium. There is in Pilidium (fig. 232 A) a thickening of epiblast at the summit of the dorsal dome, which might seem, from the analogy of Mitraria, etc. (fig. 233), to correspond to the thickening of the præoral lobe, which gives rise to the supraoesophageal ganglion; but, as a matter of fact, this part of the larva does not apparently enter into the formation of the young Nemertine (fig. 232). The peculiar metamorphosis, which takes place in the development of the Nemertine out of the Pilidium[145], may, perhaps, eventually supply an explanation of this fact; but at present it remains as a still unsolved difficulty.
[145] Vide Vol. II. p. 204.
The position of the flagellum in Pilidium, and of the supraoesophageal ganglion in Mitraria, suggests a different view of the origin of the supraoesophageal ganglion from that adopted above. The position of the ganglion in Mitraria corresponds closely with that of the auditory organ in Ctenophora; and it is not impossible that the two structures may have had a common origin. If this view is correct, we must suppose that the apex of the aboral lobe has become the centre of the præoral field of the Pilidium and Trochosphere larval forms[146]--a view which fits in very well with their structure (figs. 226 and 233). The whole of the questions concerning the nervous system are still very obscure, and until further facts are brought to light no definite conclusions can be arrived at.
[146] The independent development of the supraoesophageal ganglion and ventral nerve-cord in Chætopoda (vide Kleinenberg, Development of Lumbricus trapezoides) agrees very satisfactorily with this view.
The absence of sense organs on the præoral lobe of larval Echinodermata, coupled with the structure of the nervous system of the adult, points to the conclusion that the adult Echinodermata have retained, and not, as is now usually held, secondarily acquired, their radial symmetry; and if this is admitted it follows that the obvious bilateral symmetry of Echinoderm larvæ is a secondary character.
[FIG. 232. A. PILIDIUM WITH AN ADVANCED NEMERTINE WORM. B. RIPE EMBRYO OF NEMERTES IN THE POSITION IT OCCUPIES IN PILIDIUM. (Both after Bütschli.)
oe. oesophagus; st. stomach; i. intestine; pr. proboscis; lp. lateral pit (cephalic sack); an. amnion; n. nervous system.]
The bilateral symmetry of many Coelenterate larvæ (the larva of Æginopsis, of many Acraspeda, of Actinia, &c.), coupled with the fact that a bilateral symmetry is obviously advantageous to a free-swimming form, is sufficient to shew that this supposition is by no means extravagant; while the presence of only two alimentary diverticula in Echinoderm larvæ is quite in accord with the presence of a single pair of perigastric chambers in the early larva of Actinia, though it must be admitted that the derivation of the water-vascular system from the left diverticulum is not easy to understand on this view.
A difficulty in the above speculation is presented by the fact of the anus of the Echinodermata being the permanent blastopore, and arising prior to the mouth. If this fact has any special significance, it becomes difficult to regard the larva of Echinoderms and that of the other types as in any way related; but if the views already urged, in a previous section on the germinal layers, as to the unimportance of the blastopore, are admitted, the fact of the anus coinciding with the blastopore ceases to be a difficulty. As may be seen, by referring to fig. 231 C, the anus is placed on the dorsal side of the ciliated band. This position for the anus adapts itself to the view that the Echinoderm larva had originally a radial symmetry, with the anus placed at the aboral apex, and that, with the elongation of the larva on the attainment of a bilateral symmetry, the aboral apex became shifted to the present position of the anus.
It may be noticed that the obscure points connected with the absence of a body cavity in most adult Platyelminthes, which have already been dealt with in the section of this chapter devoted to the germinal layers, present themselves again here; and that it is necessary to assume either that alimentary diverticula, like those in the Echinodermata, were primitively present in the Platyelminthes, but have now disappeared from the ontogeny of this group, or that the alimentary diverticula have not become separated from the alimentary tract.
So far the conclusion has been reached that the archetype of the six types of larvæ had a radiate form, and that amongst existing larvæ it is most nearly approached in general shape and in the form of the alimentary canal by the Pilidium group, and in certain other particulars by the Echinoderm larvæ.
The edge of the oral disc of the larval archetype was probably armed with a ciliated ring, from which the ciliated ring of the Pilidium type and of the Echinodermata was most likely derived. The ciliated ring of the Pilidium varies greatly in its characters, and has not always the form of a complete ring. In Pilidium proper (fig. 232 A) it is a simple ring surrounding the edge of the oral disc. In Müller's larva of Thysanozoon (fig. 222 B) it is inclined at an axis to the oral disc, and might be called præoral, but such a term cannot be properly used in the absence of an anus.
[FIG. 233. TWO STAGES IN THE DEVELOPMENT OF MITRARIA. (After Metschnikoff.)
m. mouth; an. anus; sg. supraoesophageal ganglion; br. and b. provisional bristles; pr.b. præoral ciliated band.]
[FIG. 234. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.)
m. mouth; a´. anus; f.g. foot gland; x. problematical body (probably a bud). The aboral apex is turned downwards.]
The Echinoderm ring is oblique to the axis of the body, and, owing to the fact of its passing ventrally in front of the anus, must be called postoral.
The next point to be considered is that of the affinities of the other larval types to these two types.
The most important of all the larval types is the Trochosphere, and this type is undoubtedly more closely related to the Pilidium than to the Echinoderm larva. Mitraria amongst the Chætopods (fig. 233) has, indeed, nearly the form of a Pilidium, and mainly differs from a Pilidium in the possession of an anus and of provisional bristles; the same may be said of Cyphonautes (fig. 234) amongst the Polyzoa.
The existence of these two forms appears to shew that the præoral ciliated ring of the Trochosphere may very probably be derived directly from the circumoral ciliated ring of the Pilidium; the other ciliated rings or patches of the Trochosphere having a secondary origin.
The larva of the Brachiopoda (fig. 220), in spite of its peculiar characters, is, in all probability, more closely related to the Chætopod Trochosphere than to any other larval type. The most conspicuous point of agreement between them is, however, the possession in common of provisional setæ.
Echinoderm larvæ differ from the Trochosphere, not only in the points already alluded to, but in the character of the ciliated band. The Echinoderm band is longitudinal and postoral. As just stated, there is reason to think that the præoral band of the Trochosphere and the postoral band of the Echinoderm larva are both derived from a ciliated ring surrounding the oral disc of the prototype of these larvæ (vide fig. 231). In the case of the Echinodermata the anus must have been formed on the dorsal side of this ring, and in the case of the Trochosphere on the ventral side; and so the difference in position between the two rings was brought about. Another view with reference to these rings has been put forward by Gegenbaur and Lankester, to the effect that the præoral ring of the Trochosphere is derived from the breaking up of the single band of most Echinoderm larvæ into the two bands found in Bipinnaria (vide fig. 223) and the atrophy of the posterior band. There is no doubt a good deal to be said for this origin of the præoral ring, and it is strengthened by the case of Tornaria; but the view adopted above appears to me more probable.
Actinotrocha (fig. 230) undoubtedly resembles more closely Echinoderm larvæ than the Trochosphere. Its ciliated ring has Echinoderm characters, and the growth along the line of the ciliated ring of a series of arms is very similar to what takes place in many Echinoderms. It also agrees with the Echinoderm larvæ in the absence of sense organs on the præoral lobe.
Tornaria (fig. 229) cannot be definitely united either with the Trochosphere or with the Echinoderm larval type. It has important characters in common with both of these groups, and the mixture of these characters renders it a very striking and well-defined larval form.
Phylogenetic conclusions. The phylogenetic conclusions which follow from the above views remain to be dealt with. The fact that all the larvæ of the groups above the Coelenterata can be reduced to a common type seems to indicate that all the higher groups are descended from a single stem.
Considering that the larvæ of comparatively few groups have persisted, no conclusions as to affinities can be drawn from the absence of a larva in any group; and the presence in two groups of a common larval form may be taken as proving a common descent, but does not necessarily shew any close affinity.
There is every reason to believe that the types with a Trochosphere larva, viz. the Rotifera, the Mollusca, the Chætopoda, the Gephyrea, and the Polyzoa, are descended from a common ancestral form; and it is also fairly certain there was a remote ancestor common to these forms and to the Platyelminthes. A general affinity of the Brachiopoda with the Chætopoda is more than probable. All these types, together with various other types which are nearly related to them, but have not preserved an early larval form, are descended from a bilateral ancestor. The Echinodermata, on the other hand, are probably directly descended from a radial ancestor, and have more or less completely retained their radial symmetry. How far Actinotrocha[147] is related to the Echinoderm larvæ cannot be settled. Its characters may possibly be secondary, like those of the mesotrochal larvæ of Chætopods, or they may be due to its having branched off very early from the stock common to the whole of the forms above the Coelenterata. The position of Tornaria is still more obscure. It is difficult, in the face of the peculiar water-vascular vesicle with a dorsal pore, to avoid the conclusion that it has some affinities with the Echinoderm larvæ. Such affinities would seem, on the lines of speculation adopted in this section, to prove that its affinities to the Trochosphere, striking as they appear to be, are secondary and adaptive. From this conclusion, if justified, it would follow that the Echinodermata and Enteropneusta have a remote ancestor in common, but not that the two groups are in any other way related.
[147] It is quite possible that Phoronis is in no way related to the other Gephyrea.
General conclusions and summary. Starting from the demonstrated fact that the larval forms of a number of widely separated types above the Coelenterata have certain characters in common, it has been provisionally assumed that the characters have been inherited from a common ancestor; and an attempt has been made to determine (1) the characters of the prototype of all these larvæ, and (2) the mutual relations of the larval forms in question. This attempt started with certain more or less plausible suggestions, the truth of which can only be tested by the coherence of the results which follow from them, and their capacity to explain all the facts.
The results arrived at may be summarised as follows:
1. The larval forms above the Coelenterata may be divided into six groups enumerated on pages 370 to 373.
2. The prototype of all these groups was an organism something like a Medusa, with a radial symmetry. The mouth was placed in the centre of a flattened ventral surface. The aboral surface was dome-shaped. Round the edge of the oral surface was a ciliated ring, and probably a nervous ring provided with sense organs. The alimentary canal was prolonged into two or more diverticula, and there was no anus.
3. The bilaterally symmetrical types were derived from this larval form by the larva becoming oval, and the region in front of the mouth forming a præoral lobe, and that behind the mouth growing out to form the trunk. The aboral dome became the dorsal surface.
On the establishment of a bilateral symmetry the anterior part of the nervous ring gave rise (?) to the supraoesophageal ganglia, and the optic organs connected with them; while the posterior part of the nerve-ring formed (?) the ventral nerve-cords. The body cavity was developed from two of the primitive alimentary diverticula.
The usual view that radiate forms have become bilateral by the elongation of the aboral dome into the trunk is probably erroneous.
4. Pilidium is the larval form which most nearly reproduces the characters of the larval prototype in the course of its conversion into a bilateral form.
5. The Trochosphere is a completely differentiated bilateral form, in which an anus has become developed. The præoral ciliated ring of the Trochosphere is probably directly derived from the ciliated ring of Pilidium, which is itself the original ring of the prototype of all these larval forms.
6. Echinoderm larvæ, in the absence of a nerve-ganglion or special organs of sense on the præoral lobe, and in the presence of alimentary diverticula, which give rise to the body cavity, retain some characters of the prototype larva which have been lost in Pilidium. The ciliated ring of Echinoderm larvæ is probably derived directly from that of the prototype by the formation of an anus on the dorsal side of the ring. The anus was very probably originally situated at the aboral apex.
Adult Echinoderms have probably retained the radial symmetry of the forms from which they are descended, their nervous ring being directly derived from the circular nervous ring of their ancestors. They have not, as is usually supposed, secondarily acquired their radial symmetry. The bilateral symmetry of the larva is, on this view, secondary, like that of so many Coelenterate larvæ.
7. The points of similarity between Tornaria and (1) the Trochosphere and (2) the Echinoderm larvæ are probably adaptive in the one case or the other; and, while there is no difficulty in believing that those to the Trochosphere are adaptive, the presence of a water-vascular vesicle with a dorsal pore renders probable a real affinity with Echinoderm larvæ.
8. It is not possible in the present state of our knowledge to decide how far the resemblances between Actinotrocha and Echinoderm larvæ are adaptive or primary.
BIBLIOGRAPHY.
(257) Allen Thomson. British Association Address, 1877.
(258) A. Agassiz. "Embryology of the Ctenophoræ." Mem. Amer. Acad. of Arts and Sciences, Vol. X. 1874.
(259) K. E. von Baer. Ueb. Entwicklungsgeschichte d. Thiere. Königsberg, 1828-1837.
(260) F. M. Balfour. "A Comparison of the Early Stages in the Development of Vertebrates." Quart. Journ. of Micr. Sci., Vol. XV. 1875.
(261) C. Claus. Die Typenlehre u. E. Haeckel's sg. Gastræa-theorie. Wien, 1874.
(262) C. Claus. Grundzüge d. Zoologie. Marburg und Leipzig, 1879.
(263) A. Dohrn. Der Ursprung d. Wirbelthiere u. d. Princip des Functionswechsels. Leipzig, 1875.
(264) C. Gegenbaur. Grundriss d. vergleichenden Anatomie. Leipzig, 1878. Vide also Translation. Elements of Comparative Anatomy. Macmillan & Co., 1878.
(265) A. Götte. Entwicklungsgeschichte d. Unke. Leipzig, 1874.
(266) E. Haeckel. Studien z. Gastræa-theorie, Jena, 1897; and also Jenaische Zeitschrift, Vols. VIII. and IX. 1874-5.
(267) E. Haeckel. Schöpfungsgeschichte. Leipzig. Vide also Translation, The History of Creation. King & Co., London, 1878.
(268) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation, Anthropogeny. Kegan Paul & Co., London, 1878.
(269) B. Hatschek. "Studien üb. Entwicklungsgeschichte d. Anneliden." Arbeit. a. d. zool. Instit. d. Univer. Wien. 1878.
(270) O. and R. Hertwig. "Die Actinien." Jenaische Zeitschrift, Vols. XIII. and XIV. 1879.
(271) O. and R. Hertwig. Die Coelomtheorie. Jena, 1881[148].
(272) O. Hertwig. Die Chætognathen. Jena, 1880.
(273) R. Hertwig. Ueb. d. Bau d. Ctenophoren. Jena, 1880.
(274) T. H. Huxley. The Anatomy of Invertebrated Animals. Churchill, 1877.
(274*) T. H. Huxley. "On the Classification of the Animal Kingdom." Quart. J. of Micr. Science, Vol. XV. 1875.
(275) N. Kleinenberg. Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung. Leipzig, 1872.
(276) A. Kölliker. Entwicklungsgeschichte d. Menschen u. d. höh. Thiere. Leipzig, 1879.
(277) A. Kowalevsky. "Embryologische Studien an Würmern u. Arthropoden." Mém. Acad. Pétersbourg, Series VII. Vol. XVI. 1871.
(278) E. R. Lankester. "On the Germinal Layers of the Embryo as the Basis of the Genealogical Classification of Animals." Ann. and Mag. of Nat. Hist. 1873.
(279) E. R. Lankester. "Notes on Embryology and Classification." Quart. Journ. of Micr. Sci., Vol. XVII. 1877.
(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwämme." Zeit. f. wiss. Zool., Vol. XXIV. 1874.
(281) E. Metschnikoff. "Spongiologische Studien." Zeit. f. wiss. Zool., Vol. XXXII. 1879.
(282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines of Comparative Embryology. Holt and Co., New York, 1876.
(283) C. Rabl. "Ueb. d. Entwick. d. Malermuschel." Jenaische Zeitsch., Vol. X. 1876.
(284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschnecke (Planorbis)." Morph. Jahrbuch, Vol. V. 1879.
(285) H. Rathke. Abhandlungen z. Bildung und Entwicklungsgesch. d. Menschen u. d. Thiere. Leipzig, 1833.
(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig, 1829.
(287) R. Remak. Untersuch. üb. d. Entwick. d. Wirbelthiere. Berlin, 1855.
(288) Salensky. "Bemerkungen üb. Haeckels Gastræa-theorie." Archiv. f. Naturgeschichte, 1874.
(289) E. Schäfer. "Some Teachings of Development." Quart. Journ. of Micr. Science, Vol. XX. 1880.
(290) C. Semper. "Die Verwandtschaftbeziehungen d. gegliederten Thiere." Arbeiten a. d. zool.-zoot. Instit. Würzburg, Vol. III. 1876-7.
[148] This important memoir only came into my hands after this chapter was already in type.
Part II
ORGANOGENY.
Introduction
Our knowledge of the development of the organs in most of the Invertebrate groups is so meagre that it would not be profitable to attempt to treat systematically the organogeny of the whole animal kingdom.
For this reason the plan adopted in this section of the work has been to treat somewhat fully the organogeny of the Chordata, which is comparatively well known; and merely to indicate a few salient facts with reference to the organogeny of other groups. In the case of the nervous system, and of some other organs which especially lend themselves to this treatment, such as the organs of special sense and the excretory system, a wider view of the subject has been taken; and certain general principles underlying the development of other organs have also been noticed.
The classification of the organs is a matter of some difficulty. Considering the character of this treatise it seemed desirable to arrange the organs according to the layers from which they are developed. The compound nature of many organs, e.g. the eye and ear, renders it, however, impossible to carry out consistently such a mode of treatment. I have accordingly adopted a rough classification of the organs according to the layers, dropping the principle where convenient, as, for instance, in the case of the stomodæum and proctodæum.
The organs which may be regarded as mainly derived from the epiblast are (1) the skin; (2) the nervous system; (3) the organs of special sense.
Those from the mesoblast are (1) the general connective tissue and skeleton; (2) the vascular system and body cavity; (3) the muscular system; (4) the urinogenital system.
Those from the hypoblast are the alimentary tract and its derivates; with which the stomodæum and proctodæum and their respective derivates are also dealt with.
BIBLIOGRAPHY.
General works dealing with the development of the organs of the Chordata.
(291) K. E. von Baer. Ueber Entwicklungsgeschichte d. Thiere. Königsberg, 1828-1837.
(292) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes. London, 1878.
(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Säugethiere U. d. Menschen. Leipzig, 1842.
(294) C. Gegenbaur. Grundriss d. vergleichenden Anatomie. Leipzig, 1878. Vide also English translation, Elements of Comp. Anatomy. London, 1878.
(295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I. London, 1874.
(296) Alex. Götte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.
(297) W. His. Untersuch. üb. d. erste Anlage d. Wirbelthierleibes. Leipzig, 1868.
(298) A. Kölliker. Entwicklungsgeschichte d. Menschen u. der höheren Thiere. Leipzig, 1879.
(299) H. Rathke. Abhandlungen ü. Bildung und Entwicklungsgeschichte d. Menschen u. d. Thiere. Leipzig, 1838.
(300) H. Rathke. Entwicklungs. d. Natter. Königsberg, 1839.
(301) H. Rathke. Entwicklungs. d. Wirbelthiere. Leipzig, 1861.
(302) R. Remak. Untersuchungen üb. d. Entwicklung d. Wirbelthiere. Berlin, 1850-1855.
(303) S. L. Schenk. Lehrbuch d. vergleich. Embryologie d. Wirbelthiere. Wien, 1874.
XIV. THE EPIDERMIS AND ITS DERIVATIVES
In many of the Coelenterata the outermost layer of the blastoderm is converted as a whole into the skin or ectoderm. The cells composing it become no doubt in part differentiated into muscular elements and in part into nervous elements, &c.; but still it may remain through life as a simple external membrane. This membrane contains in itself indefinite potentialities for developing into various organs, and in all the true Triploblastica these potentialities are more or less realized. The embryonic epiblast ceases in fact, in the higher forms, to become converted as a whole into the epidermis, but first gives rise to parts of the nervous system, organs of special sense, and other parts.
After the formation of these parts the remnant of the epiblast gives rise to the epidermis, and often unites more or less intimately with a subjacent layer of mesoblast, known as the dermis, to form with it the skin.
Various differentiations may arise in the epidermis forming protective or skeletal structures, terminal sense organs, or glands. The structure of the epidermis itself varies greatly, and for Vertebrates its general modifications have been already sufficiently dealt with in chapter XII. Of its special differentiations those of a protective or skeletal nature and those of a glandular nature may be considered in this place.
Protective epidermal structures. These structures constitute a general cuticle or an exoskeleton of scales, hairs, feathers, nails, hoofs, &c. They may be entirely formed from the epidermis either as (1) a cuticular deposit, or as (2) a chitinization, a cornification, or calcification of its constituent cells. These two processes run into each other, and are in many cases not easily distinguished. The protective structures of the epidermis may be divided into two groups according as they are formed on the outer or the inner side of the epidermis. Dermal skeletal structures are in many cases added to them. Amongst the Invertebrata the most widely distributed type of exoskeleton is a cuticle formed on the outer surface of the epidermis, which reaches its highest development in the Arthropoda. In the same class with this cuticle must be placed the molluscan and brachiopod shells, which are developed as cuticular plates on special regions of the epidermis. They differ, however, from the more usual form of cuticle in their slighter adhesion to the subjacent epidermis, and in their more complicated structure. The test of Ascidians is an abnormal form of exoskeleton belonging to this type. It is originally formed (Hertwig and Semper) as a cuticle on the surface of the epidermis; but subsequently epidermic cells migrate into it, and it then constitutes a tissue similar to connective tissue, but differing from ordinary epidermic cuticles in that the cells which deposit it do so over their whole surface, instead of one surface, as is usually the case with epithelial cells.
In the Vertebrata the two types of exoskeleton mentioned above are both found, but that developed on the inner surface of the epidermis is always associated with a dermal skeleton, and that on the outer side frequently so. The type of exoskeleton developed on the inner side of the general epidermis is confined to the Pisces, where it appears as the scales; but a primitive form of these structures persists as the teeth in the Amphibia and Amniota. The type developed on the outer side of the epidermis is almost entirely[149] confined to the Amphibia and Amniota, where it appears as scales, feathers, hairs, claws, nails, &c. For the histological details as to the formation of these various organs I must refer the reader to treatises on histology, confining my attention here to the general embryological processes which take place in their development.
[149] The horny teeth of the Cyclostomata are structures belonging to this group.
The most primitive form of the first type of dermal structures is that of the placoid scales of Elasmobranchii[150]. These consist, when fully formed, of a plate bearing a spinous projection. They are constituted of an outer enamel layer on the projecting part, developed as a cuticular deposit of the epidermis (epiblast), and an underlying basis of dentine (the lower part of which may be osseous) with a vascular pulp in its axis. The development (fig. 235) is as follows (Hertwig, No. 306). A papilla of the dermis makes its appearance, the outer layer of which gradually calcifies to form the dentine and osseous tissue. This papilla is covered by the columnar mucous layer of the epidermis (e), from which it is separated by a basement membrane, itself a product of the epidermis. This membrane gradually thickens and calcifies, and so gives rise to the enamel cap (o). The spinous point gradually forces its way through the epidermis, so as to project freely at the surface.
[150] For the most important contributions on this subject from which the facts and views here expressed are largely derived, vide O. Hertwig, Nos. 306-308.
[FIG. 235. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYONIC SHARK, TO SHEW A DEVELOPING PLACOID SCALE. (From Gegenbaur; after O. Hertwig.)
E. epidermis; C. layers of dermis; d. uppermost layer of dermis; p. papilla of dermis; e. mucous layer of epidermis; o. enamel layer.]
The scales of other forms of fishes are to be derived from those of Elasmobranchii. The great dermal plates of many fishes have been formed by the concrescence of groups of such scales. The dentine in many cases partially or completely atrophies, leaving the major part of the scale formed of osseous tissue; such plates often become parts of the internal skeleton.
The teeth, as will be more particularly described in the section on the alimentary tract, are formed by a modification of the same process as the placoid scales, in which a ridge of the epithelium grows inwards to meet a connective tissue papilla, so that the development of the teeth takes place entirely below the superficial layer of epidermis.
In most Teleostei the enamel and dentine layers have disappeared, and the scales are entirely formed of a peculiar calcified tissue developed in the dermis.
The cuticle covering the scales of Reptiles is the simplest type of protective structure formed on the outer surface of the epidermis. The scales consist of papillæ of the dermis and epidermis; and are covered by a thickened portion of a two-layered cuticle, formed over the whole surface of the body from a cornification of the superficial part of the epidermis. Dermal osseous plates may be formed in connection with these scales, but are never of course united with the superficial cuticle.
Feathers are probably special modifications of such scales. They arise from an induration of the epidermis of papillæ containing a vascular core. The provisional down, usually present at the time of hatching, is formed by the cornification of longitudinal ridges of the mucous layer of the epidermis of the papillæ; each cornified ridge giving rise to a barb of the feather. The horny layer of the epidermis forms a provisional sheath for the developing feather below. When the barbs are fully formed this sheath is thrown off, the vascular core dries up, and the barbs become free except at their base.
Without entering into the somewhat complicated details of the formation of the permanent feathers, it may be mentioned that the calamus or quill is formed by a cornification in the form of a tube of both layers of the epidermis at the base of the papilla. The quill is open at both ends, and to it is attached the vexillum or plume of the feather. In a typical feather this is formed at the apex of the papilla from ridge-like thickenings of the mucous layer of the epidermis, arranged in the form of a longitudinal axis, continuous with the cornified mucous layer of the quill, and from lateral ridges. These subsequently become converted into the axis and barbs of the plume. The external epidermic layer becomes converted into a provisional horny sheath for the true feather beneath.
On the completion of the plume of the feather the external sheath is thrown off, leaving it quite free, and the vascular core belonging to it shrivels up. The papilla in which the feather is formed becomes at a very early period secondarily enveloped in a pit or follicle which gradually deepens as the development of the feather is continued.
Hairs (Kölliker, No. 298) are formed in solid processes of the mucous layer of the epidermis, which project into the subjacent dermis. The hair itself arises from a cornification of the cells of the axis of one of the above processes; and is invested by a sheath similarly formed from the more superficial epidermic cells. A small papilla of the dermis grows into the inner end of the epidermic process when the hair is first formed. The first trace of the hair appears close to this papilla, but soon increases in length, and when the end of the hair projects from the surface, the original solid process of the epidermis becomes converted into an open pit, the lumen of which is filled by the root of the hair. Hairs differ in their mode of formation from scales in a manner analogous to that in which the teeth differ from ordinary placoid scales; i.e. they are formed in inwardly directed projections of the epidermis instead of upon free papillæ at the surface.
Nails (Kölliker, No. 298) are developed on special regions of the epidermis, known as the primitive nail beds. They are formed by the cornification of a layer of cells which makes its appearance between the horny and mucous layers of the epidermis. The distal border of the nail soon becomes free, and the further growth is effected by additions to the under side and attached extremity of the nail.
Although the nail at first arises in the interior of the epidermis, yet its position on the outer side of the mucous layer clearly indicates with which group of epidermic structures it should be classified.
Dermal skeletal structures. We have seen that in the Chordata skeletal structures, which were primitively formed of both an epidermic and dermic element, may lose the former element and be entirely developed in the dermis. Amongst the Invertebrata there are certain dermal skeletal structures which are evolved wholly independently of the epidermis. The most important of these structures are the skeletal plates of the Echinodermata.
Glands. The secretory part of the various glandular structures belonging to the skin is invariably formed from the epidermis. In Mammalia it appears that these glands are always formed as solid ingrowths of the mucous layer (Kölliker, No. 298). The ends of these ingrowths dilate to form the true glandular part of the organs, while the stalks connecting the glandular portions with the surface form the ducts. In the case of the sweat-glands the lumen of the duct becomes first established. Its formation is inaugurated by the appearance of the cuticle, and appears first at the inner end of the duct and thence extends outwards (Ranvier, No. 311). In the sebaceous glands the first secretion is formed by a fatty modification of the whole of the central cells of the gland.
The muscular layer of the secreting part of the sweat-glands is formed, according to Ranvier (No. 311), from a modification of the deeper layer of the epidermic cells.
The Mammary Glands arise in essentially the same manner as the other glands of the skin[151]. The glands of each side are formed as a solid bud of the mucous layer of the epidermis. From this bud processes sprout out, each of which gives rise to one of the numerous glands of which the whole organ is formed. Two very distinct types in the relation of the ducts of the glands to the nipple are found (Gegenbaur, No. 313).
BIBLIOGRAPHY OF EPIDERMIS.
General.
(304) T. H. Huxley. "Tegumentary organs." Todd's Cyclopædia of Anat. and Physiol.
(305) P. Z. Unna. "Histol. u. Entwick. d. Oberhaut." Archiv f. mikr. Anat. Vol. XV. 1876. Vide also Kölliker (No. 298).
Scales of the Pisces.
(306) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d. Zähne d. Selachier." Jenaische Zeitschrift, Vol. VIII. 1874.
(307) O. Hertwig. "Ueber d. Hautskelet d. Fische." Morphol. Jahrbuch, Vol. II. 1876. (Siluroiden u. Acipenseridæ.)
(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." Morph. Jahrbuch, Vol. V. 1879.
Feathers.
(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873.
(310) Th. Studer. "Beiträge z. Entwick. d. Feder." Zeit. f. wiss. Zool., Vol. XXX. 1878.
Sweat-glands.
(311) M. S. Ranvier. "Sur la structure des glandes sudoripares." Comptes Rendus, Dec. 29, 1879.
Mammary glands.
(312) C. Creighton. "On the development of the Mamma and the Mammary function." Jour. of Anat. and Phys., Vol. XI. 1877.
(313) C. Gegenbaur. "Bemerkungen üb. d. Milchdrüsen-Papillen d. Säugethiere." Jenaische Zeit., Vol. VII. 1873.
(314) M. Huss. "Beitr. z. Entwick. d. Milchdrüsen b. Menschen u. b. Wiederkäuern." Jenaische Zeit., Vol. VII. 1873.
(315) C. Langer. "Ueber d. Bau u. d. Entwicklung d. Milchdrüsen." Denk. d. k. Akad. Wiss. Wien, Vol. III. 1851.
[151] For a very different view on this subject vide Creighton (No. 312).