Microscopical Researches Into the Accordance in the Structure and Growth of Animals and Plant

Front matter

THE

SYDENHAM SOCIETY

INSTITUTED

Introduction

MICROSCOPICAL RESEARCHES

&c. &c.

INTRODUCTION.

ALTHOUGH plants present so great a variety of external form, yet they are no less remarkable for the simplicity of their internal structure. This extraordinary diversity in figure is produced solely by different modes of junction of simple elementary structures, which, though they present various modifications, are yet throughout essentially the same, namely, cells. The entire class of the Cellular plants consists only of cells; many of them are formed solely of homogeneous cells strung together, some of even a single cell. In like manner, the Vascular plants, in their earliest condition, consist merely of simple cells; and the pollen-granule, which, according to Schleiden’s discovery, is the basis of the new plant, is in its essential parts only a cell. In perfectly-developed vascular plants the structure is more complex, so that not long since, their elementary tissues were distinguished as cellular and fibrous tissue, and vessels or spiral-tubes. Researches on the structure, and particularly on the development of these tissues, have, however, shown that these fibres and spiral-tubes are but elongated cells, and the spiral-fibres only spiral-shaped depositions upon the internal surface of the cells. Thus the vascular plants consist likewise of cells, some of which only have advanced to a higher degree of development. The lactiferous vessels are the only structure not as yet reduced to cells; but further observations are required with respect to their development. According to Unger (Aphorismen zur Anatomie und Physiol. der Pflanzen, Wien, 1838, p. 14,) they in like manner consist of cells, the partition-walls of which become obliterated.

Animals, which present a much greater variety of external form than is found in the vegetable kingdom, exhibit also, and especially the higher classes in the perfectly-developed condition, a much more complex structure in their individual tissues. How broad is the distinction between a muscle and a nerve, between the latter and cellular tissue, (which agrees only in name with that of plants,) or elastic or horny tissue, and so on. When, however, we turn to the history of the development of these tissues, it appears, that all their manifold forms originate likewise only from cells, indeed from cells which are entirely analogous to those of vegetables, and which exhibit the most remarkable accordance with them in some of the vital phenomena which they manifest. The design of the present treatise is to prove this by a series of observations.

It is, however, necessary to give some account of the vital phenomena of vegetable cells. Each cell is, within certain limits, an Individual, an independent Whole. The vital phenomena of one are repeated, entirely or in part, in all the rest. These Individuals, however, are not ranged side by side as a mere Aggregate, but so operate together, in a manner unknown to us, as to produce an harmonious Whole. ‘The processes which go forward in the vegetable cells, may be reduced to the following heads: 1, the production of new cells; 2, the expansion of existing cells; 3, the transformation of the cell-contents, and the thickening of the cell-wall; 4, the secretion and absorption carried on by cells.

The excellent researches of Schleiden, which throw so much light upon this subject, form the principal basis for my more minute observations on these separate vital phenomena. (See his “ Beitrage zur Phytogenesis,” in Müller’s Archiv, 1838, p. 137, plates 3 and 4.)

First, of the production of new cells. According to Schleiden, in Phenogamous plants, this process always (except as regards the cells of the Cambium,) takes place within the already mature cells, and in a most remarkable manner from out of the well-known cell-nucleus. On account of the importance of the latter in reference to animal organization, I here introduce an abridgment of Schleiden’s description of it. A delineation is given in plate I, fig. 1, a, a, taken from the onion. This structure — named by R. Brown, Areola or cell-nucleus, by Schleiden, Cytoblast — varies in its outline between oval and circular, according as the solid which it forms passes from the lenticular into the perfectly spheroidal figure. Its colour is mostly yellowish, sometimes, however, passing into an almost silvery white; and in consequence of its transparency, often scarcely distinguishable. It is coloured by iodine, according to its various modifications, from a pale yellow to the darkest brown. Its size varies considerably, according to its age, and according to the plants, and the different parts of a plant in which it is found, from 0:0001 to 0:0022 Paris inch. Its internal structure is granular, without, however, the granules, of which it consists, being very clearly distinct from each other. Its consistence is very variable, from such a degree of softness as that it almost dissolves in water, to a firmness which bears a considerable pressure of the compressorium without alteration of form. In addition to these peculiarities of the cytoblast, already made known by Brown and Meyen, Schleiden has discovered in its interior a small corpuscle (see plate I, fig. 1, 4,) which, in the fully-developed cytoblast, looks like a thick ring, or a thick-walled hollow globule. It appears, however, to present a different appearance in different cytoblasts. Sometimes only the external sharply-defined circle of this ring can be distinguished, with a dark point in the centre,—occasionally, and indeed most frequently, only a sharply circumscribed spot. In other instances this spot is very small, and sometimes cannot be recognized at all. As it will frequently be necessary to speak of this body in the following treatise, I will for brevity’s sake name it the “nucleolus,” (Kernkorperchen, ‘nucleus-corpuscle.”) According to Schleiden, sometimes two, more rarely three, or, as he has personally informed me, even four such nucleoli occur in the cytoblast. Their size is very various, ranging from the semi-diameter of the cytoblast to the most minute point. The following is Schleiden’s description of the origin of the cells from the cytoblast. So soon as the cytoblasts have attained their full size, a delicate transparent vesicle, the young cell, rises upon their surface, and is placed upon the flat cytoblast like a watch-glass upon a watch. It is at this time so delicate that it dissolves in distilled water in a few minutes. It gradually expands, becomes more consistent, and at length so large, that the cytoblast appears only as a small body inclosed in one of the side walls. The portion of the cell-wall which covers the cytoblast on the inner side, is, however, extremely delicate and gelatinous, and only in rare instances to be observed; it soon undergoes absorption together with the cytoblast, which like-wise becomes absorbed in the fully-developed cell. The cytoblasts are formed free within a cell, in a mass of mucus-granules, and the young cells lie also free in the parent cell, and assume, as they become flattened against each other, the polyhedral form. Subsequently the parent cell becomes absorbed. (See a delineation of young cells within parent cells, plate I, fig. 2, b, b, b.) It cannot at present be stated with certainty that the formation of new cells always takes place from a cystoblast, and always within the existing cells, for the Cryptogamia have not as yet been examined in this respect, nor has Schleiden yet expressed his views in reference to the Cambium. Moreover, according to Mirbel, a formation of new cells on the outside of the previous ones takes place in the intercellular canals and on the surface of the plant in the Phanerogamia. (See Mirbel on “Marchantia,” in Annales du Musée, 1, 55; and the counter-observations of Schleiden, Müller’s Archiv, 1838, p. 161.) A mode of formation of new cells, different from the above described, is exhibited in the multiplication of cells by division of the existing ones; in this case partition-walls grow across the old cell, if, as Schleiden supposes, this be not an illusion, inasmuch as the young cells might escape observation in consequence of their transparency, and at a later stage, their line of contact would be regarded as the partition wall of the parent cell.

The expansion of the cell when formed, is, either regular on all sides, in which case it remains globular, or it becomes polyhedral from flattening against the neighbouring cells, or it is irregular from the cell growing more vigorously in one or in several directions. What was formerly called the fibrous tissue, which contains remarkably elongated cells, is formed in this manner. These fibres also become branched, when different points of the cell-wall expand in different directions. This expansion of the cell-wall cannot be explained as a merely mechanical effect, which would continually tend to render the cell-membrane thinner. It is often even combined with a thickening of the cell-wall, and is probably effected by that process of nutrition called intussusception. (See Hugo Mohl’s “Erläuterung und Vertheidigung meiner Ansicht von der Structur der Pflanzen substanzen,” Tübingen, 1836.) The flattening of the cells may also be ascribed to the same cause.

With regard to the changes which the cell-contents and cell-wall undergo during vegetation, I only take into consideration the thickening of the latter, as I have but a few isolated observations upon the transformations of the contents of animal cells, which however indicate analogous changes to those of plants. The thickening of the cell-walls takes place, either by the deposition from the original wall, of substances differing from, or more rarely, homogeneous with it, upon the internal surface of the cell, or by an actual thickening of the substance of the cell- wall. The first-mentioned form of deposition occurs in strata, at least this may be distinctly seen in many situations. (See Meyen’s Pflanzen-Physiologie. Bd. 1, tab. I, fig. 4.) Very frequently,—according to Valentin, universally,—these depositions take place in spiral lines; this is very distinct, for example, in the spiral canals and spiral cells. The thickening of the cell-membrane itself, although more rare, appears still in some instances indubitable, for instance, in the pollen-tubes, (e. g. Phormium tenax.) Probably that extremely remarkable phenomenon of the motion of the fluid, which has now been observed in a great many cells of plants, is connected with the transformation of the cell-contents. In the Charæ, in which it is most distinct, a spiral motion may also be recognized in it. But, for the most part, the currents intersect each other in the most complex manner.

Absorption and Secretion may be classed as external operations of the vegetable cells. The disappearance of the parent cells in which young ones have formed, or of the cell-nucleus and of other structures, affords sufficient examples of absorption. Secretion is exhibited in the exudation of resin in the intercellular canals, and of a fluid containing sugar by the nectar-glands, &c. &c.

In all these processes each cell remains distinct, and main- tains an independent existence. Examples, however, also occur in plants, where the cells coalesce, and this not merely with regard to their walls, but the cavities also. Schleiden has found that in the Cacti, the thickened walls of several cells unite to form a homogeneous substance, in which only the remains of the cell-cavities can be distinguished. Pl. I, fig. 3, represents such a blending of the cell-walls observed by Schleiden. The entire figure is a parent cell, with thickened walls, in which four young cells have formed, the walls of which are likewise thickened and have coalesced with each other, as well as with those of the parent cell; so that only the four cavities remain with their nuclei in a homogeneous substance. The spiral vessels, and, according to Unger, the lactiferous vessels also, afford examples of the union of the cavities of several cells by the absorption of the partition walls.

After these preliminary remarks we pass on to animals. The similarity between some individual animal and vegetable tissues has already been frequently pointed out. Justly enough, however, nothing has been inferred from such individual points of resemblance. Every cell is not an analogous structure to a vegetable cell; and as to the polyhedral form, seeing that it necessarily belongs to all cells when closely compacted, it obviously is no mark of similarity further than in the circumstance of densely crowded arrangement. An analogy between the cells of animal tissues and the same elementary structure in vegetables can only be drawn with certainty in one of the following ways: either, 1st, by showing that a great portion of the animal tissues originates from, or consists of cells, each of which must have its particular wall, in which case it becomes probable that these cells correspond to the cellular elementary structure universally present in plants; or, 2dly, by proving, with regard to any one animal tissue consisting of cells, that, in addition to its cellular structure, similar forces to those of vegetable cells are in operation in its component cells; or, since this is im- possible directly, that the phenomena by which the activity of these powers or forces manifests itself, namely, nutrition and growth, proceed in the same or a similar manner in them as in the cells of plants. I reflected upon the matter in this point of view in the previous summer, when, in the course of my re- searches upon the terminations of the nerves in the tail of the Larvae of frogs (Medic. Zeitung, 1837), I not only saw the beautiful cellular structure of the Chorda Dorsalis in these larvæ, but also discovered the nuclei in the cells. J. Müller had already proved that the chorda dorsalis in fishes consists of separate cells, provided with distinct walls, and closely packed together like the pigment of the Choroid. The nuclei, which in their form are so similar to the usual flat nuclei of the vegetable cells that they might be mistaken for them, thus furnished an additional point of resemblance. As however the importance of these nuclei was not known, and since most of the cells of mature plants exhibit no nuclei, the fact led to no farther result. J. Müller had proved, with regard to the cartilage-corpuscles discovered by Purkinje and Deutsch in several kinds of cartilage, from their gradual transition into larger cells, that they were hollow, thus in a more extended sense of the word, cells; and Miescher also distinguishes an especial class of spongy cartilages of a cellular structure. Nuclei were likewise known in the cartilage-corpuscles. Müller, and subsequently Meckauer, having observed the projection of the cartilage-corpuscles at the edge of a preparation, it became very probable that at least some of them must be considered as cells in the restricted sense of the word, or as cavities inclosed by a membrane. Gurlt also, when describing one form of permanent cartilage, calls them vesicles. I next succeeded in actually observing the proper wall of the cartilage-corpuscles, first in the branchial cartilages of the frog’s larvæ, and subsequently also in the fish, and also the accordance of all cartilage-corpuscles, and by this means in proving a cellular structure, in the restricted sense of the word, in all cartilages. During the growth of some of the cartilage-cells, a thickening of the cell-walls might also be perceived.

Thus was the similarity in the process of vegetation of animal and vegetable cells still further developed. Dr. Schleiden opportunely communicated to me at this time his excellent researches upon the origin of new cells in plants, from the nuclei within the parent-cell. The previously enigmatical contents of the cells in the branchial cartilages of the frog’s larve thus became clear to me; I now recognized in them young cells, provided with a nucleus. Meckauer and Arnold had already found fat-vesicles in the cartilage-corpuscles. As I soon afterwards suc- ceeded in rendering the origin of young cells from nuclei within the parent-cells in the branchial cartilages very probable, the matter was decided. Cells presented themselves in the anima body having a nucleus, which in its position with regard to the cell, its form and modifications, accorded with the cytoblast of vegetable cells, a thickening of the cell-wall took place, and the formation of young cells within the parent-cell from a similar cytoblast, and the growth of these without vascular connexion was proved. This accordance was still farther shown by many details; and thus, so far as concerned these individual tissues, the desired evidence, that these cells correspond to the elementary cells of vegetables was furnished. I soon conjectured that the cellular formation might be a widely extended, perhaps a universal principle for the formation of organic substances. Many cells, some having nuclei, were already known; for example, in the ovum, epithelium, blood-corpuscles, pigment, &c. &c. It was an easy step in the argument to comprise these recognized cells under one point of view; to compare the blood-corpuscles, for example, with the cells of epithelium, and to consider these, as likewise the cells of cartilages and vegetables, as corresponding with each other, and as realizations of that common principle. This was the more probable, as many points of agreement in the progress of development of these cells were already known. C. H. Schultz had already proved the preexistence of the nuclei of the blood-corpuscles, the formation of the vesicle around the same, and the gradual expansion of this vesicle. Henle had observed the gradual increase in size of the epidermal cells from the under layers of the epidermis, towards the upper ones. The growth of the germinal vesicle, observed by Purkinje, served also at first as an example of the growth of one cell within another, although it afterwards became more probable that it had not the signification of a cell, but of a cell-nucleus, and thus furnished proof that everything having the cellular form does not necessarily correspond to the cells of plants. A precise term for these cells, which correspond to those of plants, should be adopted; either elementary cells, or vegetative cells (vegetations-zellen). By still further examination, I constantly found this principle of cellular formation more fully realized. The germinal membrane was soon discovered to be composed entirely of cells, and shortly afterwards cell-nuclei, and subsequently also cells were found in all tissues of the animal body at their origin; so that all tissues consist of cells, or are formed by various modes, from cells. The other proof of the analogy between animal and vegetable cells was thus afforded.

I shall follow the same course in communicating the separate observations, and shall speak, therefore, in the next place of the structure and growth of the chorda dorsalis and cartilage, and in the second section treat of the germinal membrane and the remaining tissues.

SECTION I

SECTION I.

ON THE STRUCTURE AND GROWTH OF THE CHORDA DORSALIS AND CARTILAGE.

1. Chorda Dorsalis.

THE Chorda Dorsalis in the larve of frogs and fishes lies in, or in some instances, under the bodies of the vertebra, and is continued behind the coccyx, through the whole length of the tail. It is inclosed by a firm sheath, and forms a spindle-like, consistent, gelatiniform, transparent cord, which is thickest at the commencement of the tail, and thence gradually diminishes in each direction, both towards the skull and the point of the tail. It cannot well be separated entire in recently killed animals, but is best obtained from them in the form of delicate transverse sections. If the animal be placed in water for twenty-four hours or longer after death, and the tail then severed from the body at their point of junction, the chorda dorsalis may be entirely pressed out, by gently scraping along its course from the point of the tail, or from the head, towards the wound. As this does not succeed if the animal be allowed to let out of water for the same period after death, the easier separableness appears to depend upon a penetration of the water between the chorda dorsalis and its sheath; the firmer connexion of it in the fresh condition, however, only upon a more close contact, or wedging in of the chorda dorsalis, and not upon a vascular connexion, for I do not suppose that it contains any vessels. Microscopically examined, it exhibits, as J. Müller has discovered in fishes, a cellular structure in its interior, surrounded externally by a proportionately thin cortical substance (rinde), which is beset with scattered granules. The interior exactly resembles the parenchymatous cellular tissue of plants. (See plate I, fig. 4.) It is readily seen, especially at the point of contact of three cells, that each one is surrounded by its own proper membrane. The cells vary much in size, being usually largest in the centre, and becoming somewhat smaller towards the outside. They have an irregular polyhedral shape, mostly with spherical surfaces, which are sometimes convex towards the outside, sometimes towards the cavity of the cell. Their walls are very thin, colourless, smooth, and almost completely transparent, firm, and slightly extensible. They dissolve readily in caustic potash. The rudiments of the chorda dorsalis in the conical interstices of the vertebrae of cartilaginous fishes are not dissolved by dilute or concentrated acetic acid. The chorda dorsalis of fishes according to J. Müller does not become converted into gelatine after long boiling.

The cells of the chorda dorsalis of frog’s larvee contain in their interior a colourless, homogeneous, transparent fluid, which does not become cloudy at a boiling heat; the slight clouding observed in the chorda dorsalis after boiling, appears to be situated more in the cell-walls, which afterwards appear minutely granulated.

In the larva of Pelobates fuscus another formation occurs, inasmuch as by far the greater proportion of these cells containv a very distinct nucleus. It has the appearance of a somewhat yellowish-coloured small disc, of a roundish oval form, rather smaller than a blood-corpuscle of the frog, and almost as flat. (See plate I, fig. 4a, where it is represented from the chorda dorsalis of Cyprinus erythrophthalmus.) In frog’s larvae the nucleus is nearly twice as large. It has a sharp, dark margin, and appears minutely granulated. In this little disc may be seen one, rarely two, and very seldom three dark, sharply circumscribed spots. It thus entirely resembles, both as a whole as well as in its modifications, the cytoblast of vegetable cells with its nucleolus, and microscopically, cannot at all be distinguished from it. Compare plate I, fig. 4a, with plate I, fig. 1a. But it also corresponds with it in its position in the cell. In very many cells, the vertical wall of which is viewed from above, it may be seen that the nucleus lies close on the inner surface of the wall of the cell, or even embedded in the wall. It appears then, as in plate I, fig. 1 a, only still somewhat flatter. I have not, however, succeeded in observing that a lamella of the cell-wall passes over its internal surface, which is also but rarely seen in plants. If the external minutely granulated cortical substance of the chorda dorsalis of Pelobates fuscus be more accurately examined, it is found that the granules are oval, and furnished with a nucleolus, and that, with the exception of their being only about half as large, they entirely resemble the cell-nuclei. This cortical substance is not sharply separated from the proper tissue of the chorda dorsalis; and as the cells of the latter suddenly diminish very much towards the cortical substance, I think that these granules upon the latter are the cytoblasts of flattened cells which form it. Sometimes, although but indistinctly even with a very favorable light, very fine lines may be perceived in the intermediate spaces between these granules, where the cells come in contact, as in the common tabular (or scaly) epithelium. In the chorda dorsalis of the larva of Rana esculenta, where the nuclei in the cells are not distinct, these nuclei in the cortical substance are not seen; the tabular structure, however, is evident in them. One must be very cautious in denying the presence of the cytoblasts, when they are not immediately recognizable. They may in animals, as in plants, attain such a degree of transparency, as renders them very difficult of observation. Thus, I could not for a long time detect them in the rudiment of the chorda dorsalis, which is found in the conical intermediate spaces of the vertebra, in a large Carp, until on a very clear day they appeared very pale but quite recognizable, and of precisely the form above described. They were somewhat more distinct in the Pike and Cyprinus erythrophthalmus. The delineation, plate I, fig. 4, is taken from the latter. They are however smaller in these fishes than in frog’s larvee.

To return to the larva of Pelobates fuscus. Here the cells of the chorda dorsalis lie so close to each other, that the walls of the two neighbouring cells are in immediate contact. Even when three or more cells are in contact, they are generally so close, that only the contiguous walls are observable. Sometimes, however, in such instances, a small intermediate space remains, which is larger than could be filled up by the unthickened cell-wall; and there is then seen, as in plants, a species (apparent or real?) of intercellular substance, or an intercellular canal. With regard to this latter (intercellular canal), occasionally, though rarely, in such an instance of close contiguity of three cells, upon making a transverse section, the cell-walls are observed sharply bounded, as well towards the cell as externally, and between the cells a small triangular interstice is seen, which is filled by a transparent fluid (not by air), or at least by a substance which refracts the light in a different manner from the cell-walls, just as it is represented in plate I, fig. 1c, from the onion.

Young cells, which float free, form within the cells of the chorda dorsalis, as in plants. They are, however, in the larvae of the frog so transparent, that very favorable light and good instruments are required to see them. The number of cells, also, in which new ones are formed in the larvæ is not great, at least in such as are to be had in the latter part of autumn. In the above-mentioned species of Cyprinus, and also in other fishes, they are, however, easy to be seen, and in greater number. Vesicles of very various sizes may be perceived in the cavities of many of these cells, and also in those of the larvæ of the frog, though they are more difficult of observation in the latter; a single one of these vesicles sometimes fills the greater part of the cavity; and occasionally several lie in one cell. (Pl. I, fig. 4, b, b, c.) They are commonly quite round; but not unfrequently two are in contact, and flattened against each other. That they lie free in the cell, follows from the fact, that they may be isolated without rupture. If, for instance, a small portion of the chorda dorsalis be torn into minute pieces, and a thin plate of glass with water be placed upon them, by moving this lightly backwards and forwards a few times, some such isolated vesicles may often be brought into the field of vision. They may then be made to roll about, and thus demonstrate their globular form. I have taken great pains to discover a nucleus in their walls, but without success. The young cells of the chorda dorsalis, also, in the larvae so often mentioned, have often the appearance, so long as they are not isolated, of possessing a nucleus: but one may readily be deceived here, since such a nucleus may belong to a cell lying above or below them. Caution must also be used, not to confound a globular epithelial cell, which may have shipped into the chorda dorsalis in making the transverse section, with the true cells of that structure. I have not as yet been able, with certainty, to observe any nucleus, at least not of the characteristic form, in isolated young cells of the chorda dorsalis. In rare instances, a very small corpuscle, (d, d, of the figure,) lay in the inner surface of the young cell. It must remain a question whether the nucleus is really wanting, or whether it is only not visible in consequence of its translucency, or whether these corpuscles are developed into the nucleus. The chorda dorsalis accords with the vegetable cells, at least in this respect, that young cells are formed within the old ones.

With regard to the thickening of the cell walls; these appear to remain always simple (unchanged) in the chorda dorsalis of the larva of the frog. But in the fully developed osseous fishes, in Cyprinus, for example, a thickening is exhibited in those cells which lie near the axis of the conical interspaces of the vertebræ. The cell-cavities always become smaller in consequence of this thickening of the walls. The thickened walls, or the intermediate substance between the cell cavities consist of closely cohering longitudinal fibres, between which very fine transverse fibres are also sometimes seen. The longitudinal fibres run uninterruptedly past several cells; and the primitive membrane of each cell can no longer be distinguished.

To sum up the researches upon the chorda dorsalis in a few words; it may be said to consist of polyhedral cells, which have, in or on the internal surface of their walls, a structure, according in its form and position with the nucleus of the cells of plants, namely, an oval flat disc containing one, two, or more rarely three nucleoli. The cells usually lie in close contact with each other; but sometimes at points where three or more cells meet together, a sort of intercellular substance, or an intercellular passage is seen. Young cells, which are at first round, and float free, are formed within parent cells. Nuclei of the characteristic form, are not distinctly observed in these young cells, but sometimes a small globule lies upon their inner surface. In those cells which undergo further development, the cell-membrane ceases to exist as a distinct structure, and the intermediate substance between the cell cavities consists for the most part of longitudinal fibres.