A textbook of general embryology

Preface

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B. M.

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General embryology should occupy an important place in the collegiate study of biology. In no other connection are the essential phenomena of life better illustrated, in no other form are they more readily appreciated. The facts of embryology lead directly to the great problems of the science of biology as it exists to-day, and many fundamental biological conceptions either are directly connected with, or are illuminated by, the study of the early phenomena of individual development.

The author’s experience has clearly indicated that the subject has this value as a collegiate study. Indeed, the book is the direct outgrowth of such experience, and it has, in substance, been in use as such a text for several years. In its present form it is hoped that it will be found useful to the student who is endeavoring to comprehend the general principles of the science of life, as well as to the student preparing for the professional study of some field of biology or of medicine.

Its design as a textbook, rather than as a handbook, accounts for certain characteristics. The topics considered have throughout been approached from the standpoint of their general biological relations, and in the selection of the facts mentioned and the topics discussed, as well as in the style and method of presentation, the student has been first in mind. The arrangement of the subject matter in two sizes of type may prove useful for those undertaking a brief course. In a few instances this has involved slight repetition, but repetition is not always a pedagogic evil.

At the end of each chapter will be found a list of references to literature. Usefulness to the student has been the only criterion in determining the admission of titles to these lists. Consequently there will be found titles of works of historical importance, of recent works containing contributions of impor-

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PREFACE tance or representing present tendencies in research, and of papers containing extensive literature references, valuable illustrations, or general summaries. As far as possible the lists contain references to works presenting both, or several, sides of mooted questions mentioned in the text. There will also be found, in nearly every instance, the titles of papers from which illustrations may have been taken.

To a large extent the figures have been redrawn, from the original sources, for this work: it is a pleasure to notice the uniform courtesy with which authors have granted permission to make this use of their illustrations. The following special debts are gratefully acknowledged: to Prof. Edmund B. Wilson and The Macmillan Company, for cliches and for permission to copy a considerable number of illustrations in their “The Cell in Development and Inheritance”; to Prof. Gary N. Calkins, The Macmillan Company, and Lea and Febiger, for cliches and for permission to copy certain illustrations in their “The Protozoa” and “Protozoology”; to Prof. Ulric Dahlgren, Prof. William A. Ivepner, and The Macmillan Company, for permission to copy certain illustrations in their “Principles of Animal Histology”; to Prof. J. W. Jenkinson and the Delegates and Secretary of the Clarendon Press, for cliches from their “ Experimental Embryology”; and finally to Herr Gustav Fischer and to the several authors, for cliches and for permission to copy or otherwise make use of illustrations from Korschelt and Heider’s “Lehrbuch,” Oscar Hertwig’s “Handbuch,” Doflein’s “Protozooenkunde,” and Ziegler’s “Lehrbuch.” In every instance specific reference, both to the immediate and the ultimate sources of the figures borrowed, is made in the legends. I desire also to acknowledge my indebtedness to the authorities of The Johns Hopkins University, for the use of valuable library facilities.

W. E. K.

Baltimore, Md.,

Chapter I Ontogeny

Living organisms come into existence only as the offspring of preexisting living organisms of the same kind or species. Aristotle’s belief that eels were generated from mud and slime is represented to-day, in many youthful minds, by the firm conviction that a horse hair, if only kept long enough in water, will surely “turn to a worm.” From the time of Redi the belief that the living might be generated from the wholly non-living gradually became restricted, in its application, to lower and still lower groups of organisms. For a long time it remained applied only to those forms at the lower limit of the living — the bacteria. From this position the belief that “ spontaneous generation” of organisms occurs nowadays, was finally driven by the brilliant demonstrations of Pasteur and Tyndall that even these simplest and smallest of organisms arise only from preexisting living organisms of the same kind.

This property of producing new, specifically similar individuals is one of the few really distinctive characteristics of living things, and since the newly produced resemble closely the parent form, we speak of the property as Reproduction. The fact- that at corresponding ages offspring resemble their parents, is the fact of heredity. But when these offspring are first distinguishable as separate and new individuals they bear little or no visible resemblance to the adult organisms producing them. This resemblance appears gradually, as the result ©f a long series of processes, complex and often very special, involving changes in structure, function, and form, only at the conclusion of which has the new organism reproduced, more

l or less precisely, the form and other characteristics of its parents. The facts regarding all of these processes of development, of the external and internal changes in form and structure of the new organism, of the complex chain of processes leading to its first formation, and of the role of external factors throughout all of these, constitute the science of Embryology. We may define Embryology briefly then, as the science of the genesis of the adult organism.

As a general introduction to this subject we may first sketch in outline the broad features of reproduction among the lower animals, mentioning but a few of the almost infinitely varied forms which this process assumes. It should be understood in

Fig. 1. — Simple fission in Amceba vespertilio. After Doflein. A. Normal vegetative form. B. Commencement of fission (“biscuit form”). C. Fission nearly completed; separation of daughter cells, z, cells of an Alga, Zoochlorella.

advance that the series of reproductive processes, of increasing complexity, to be outlined, has little if any phyletic significance; this arrangement is made for comparative purposes alone.

The simplest and apparently the most primitive mode of reproduction is that known as fission, characteristic of the single-celled organisms, the Protozoa and Protophyta. In the case of simple or binary fission a separation of the nuclear material of the cell into two separate masses is followed by a constriction of the cell body into two bodies, each of which may then form parts corresponding with those carried away by its sister cell, and finally develop into a creature resembling the original organism (Fig. 1). In many unicellular forms, particularly among the Sporozoa, a process of multiple fission or brood formation occurs. This is frequently preceded by growth of the organism to an unusual size; then an uninterrupted series of simple fissions, without intermediate growth or development on the part of the daughter cells, results in the formation of a

Fig. 2. — Multiple fission in a parasitic Infusorian, Holophrya multifiliis. After Hatschek. A. Normal vegetative individual. B. Cyst containing the products of repeated binary fission; some of the zoospores are shown leaving the cyst. C. One of the zohspores, enlarged, cv, contractile vacuole; cy, cyst; m, mouth; ma, macronucleus; mi, micronucleus; t, zoospores.

large number of small organisms. These usually remain associated, frequently within a cyst formed by the parent cell, until the whole process of fission is completed (Fig. 2). In other cases the nucleus, or nuclei, alone may divide, either successively or simultaneously, into a large number of separate nuclei; the cytoplasm immediately surrounding each nucleus is then cut out as a separate cell, so that the organism appears to fragment simultaneously into a large number of small daughter organisms (Fig. 3). The number of new individuals formed in this way may vary from four, as in some Infusoria, up to as many as several hundred in many different forms, especially among the Sporozoa. When the number is large the process is more frequently termed spore formation or sporulation and the products are known as swarmspores or zoospores. Technically the term sporulation or sporogony ( metagametic division ) is used only when this process of multiple fission occurs subsequently to a process of cell fusion. If no such process of cell fusion or conjugation has preceded, the process is termed schizogony (agamogony) , and the products of the division are called schizonts {agamonts) .

Fig. 3. — Multiple fission (schizogony) in the Sporozoan, Coccidium schubergi. After Schaudinn. A. Entrance of organism (“sporozoite”) into epithelial cell of host. B, C. Two stages in growth. D. Multiple division of nucleus. E. Daughter nuclei superficial, cytoplasm still undivided. F. Fission of the superficial cytoplasm surrounding the nuclei, leaving a central undivided mass. G. Free schizonts (“ merozoites”).

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Reproduction by analogous processes also called fission, simple or multiple, is only occasional among the many-celled animals. In a few such forms the entire body separates as a unit into two or more organisms. Organs and tissues in the plane of separation are divided, and each of the daughter organisms then regenerates parts corresponding to those carried

A

C off by the other, forming new individuals smaller than the parent form, but otherwise similar to it (Fig. 4). This fission in the Metazoa is not really comparable with the similarly named process in the Protozoa; it represents a special acquirement and is usually, though not always, associated with other more complex modes of reproduction. Normal fission is known to occur in many genera among the Coelenterates, and less frequently among the Porifera, Platyhelminthes, Annulata, Bryozoa, and Echinoderms.

It is obvious that in reproduction by fission the individuality of the parent organism is lost in the act of giving rise to the new individuals, although none of the substance of the original organism perishes in the act.

j' ■ ^ss*on *n Metazoa. A, B. Two stages in the transverse fission of the Hydroid, Gonactinia prolifera. From Korschelt and Heider, after Blochmann and Huger. C. Successive transverse fissions in the Platyhelminth, Microstomum hneare After L. von Graff. 7, 77, 777, mark the levels of the successive fissions; a fourth fission also is indicated, p, p, pharynges.

In the fission of Metazoa usually many of the structures of the patent are simply transferred to the new organisms with comparatively little differentiation of parts anew, out of a visibly undifferentiated condition. In the Protozoa this ffiay or may not be the case. In some of the highly organized Ciliata most of the structural differentiation disappears just previous to fission, after which each daughter cell differentiates a typical form and structure anew (Fig. 5). In other Protozoa there is a considerable transference of characteristics accompanied by a lesser amount of regeneration or redifferentiation. After multiple fission or sporulation, the end products 6f the process are commonly minute and visibly quite unlike the adult form; this they come to resemble only through growth and differentiation, that is, through processes of true development.

A reproductive process closely allied to fission is the familiar process of budding. Here one or several small outgrowths or ‘‘buds” are produced from some portion of the parent organism, and develop into forms resembling the parent, either before or after becoming detached. This process frequently appears as a sort of unequal fission and may indeed rightly be regarded as such; but usually so great is the disparity in size, as well as in extent of differentiation, between parent and buds, that the processes are properly distinguished. Budding occurs in many Protozoa ( e.g ., Ephelota (Fig. 6, A), some Rhizopoda), and is quite frequent among the lower Metazoa, particularly in colony form-

Fig. 5. — Binary fission, in lug species. It occuis among the

the Infusorian, Eupiotesharpa. porifera Coelenterata, Platyhelmin- After Wallengren. Division ; .

stage immediately before the tlieS, Annulata, BryOZOa, and 111111- separation of the daughter cells. ^ ^ ^ ^ jn budding there

is ordinarily very little direct transference of structures, the development of the bud occurring after it has been completely delimited as a comparatively undifferentiated mass, its development may then be almost complete before the separation of the two organisms. Nor does this process involve necessarily the loss of identity of the parent, which may continue to live and produce buds for a considerable period.

In a few Protozoa, fresh water sponges, Trematodes and

Bryozoa, internal buds are formed within the parent cell or body (Fig. 7). These become free and develop into new individuals ordinarily only after the death of the parent body. Although the formation of these gemmules (Porifera), or statoblasts (Bryozoa) suggests that form of reproduction characteristic of the higher Metazoa, i.e., through internally formed germ cells, it will appear later that the two processes are not at all to be compared.

y1?’ 6— Reproduction by budding. A. In the Infusorian, Ephelola. From Calkins, Protozoa.” TV, branched macronucleus extending from the parent cell into each bud. B. In the Tunicate, Doliolum. After Neumann. Ventral outgrowth of a phorozooid. b, reproductive buds of various ages; a, germinal knob; s, stalk.

In some of the shelled Rhizopods ( Eughgplia , Arcella) a combination of the processes of budding and fission occurs. In this piocess, called bud-jission, about one-half of the protoplasm flows outside the original shell, which these forms possess, and secretes a new shell like that of the parent form; or the rudiments of the new shell may be formed before the appearance of the bud. The cell body then divides and the two similar individuals may either separate completely, or they may remain in contact, forming after repeated bud-fission, an aggregate of related though distinct organisms.

In many of the Protozoa multiple fission may be incomplete, or the daughter cells, not scattering as separate individuals, may remain associated during division and growth, forming a colony of organically related cells which as a whole is to be regarded as the individual organism. Among nearly all of these colonial or compound forms the entire colony, or cceno-

Fig. 7. — The formation and development of the statoblast, in the fresh water Bryozoan, Cristatella. A. After Braem. Others, after Verworn. A. Longitudinal section through the funiculus, showing the relations of the statoblast. B-E. Optical sections of stages in the development of the statoblast. F. Section showing rudiment of embryo, e, superficial ectoderm; i, inner layer of funiculus (ectoderm); o, outer layer of funiculus (endoderm); s, rudiment of statoblast.

bium as it is called, is not later involved as a unit in reproduction, but the component cells may individually undergo fission leading to the formation of new colonies. In forms like Pandorina or Platydorina all the cells are alike, and each reproduces, so that there may be as many new colonies formed as there are cells in the original colony (Fig. 8). In other forms the power of reproduction is limited to certain cells, termed gonidia, and there results a sharp distinction between reproductive and vegetative cells (Fig. 9). Thus in Pleodo nna, of the thirty-two cells forming the colony, four anterior cells are purely vegetative, the remaining twenty-eight are gonidial;

Fig. 8. — Reproduction in the colonial Flagellate, Pandorina morum. From Hertwig, after Pringsheim. I.’ Free-swimming vegetative colony. II. Daughter colonies formed by the fissions of each individual in a colony similar to the preceding. III. Gamete formation in a colony formed as above. IV, V, VI. Stages in the conjugation of gametes to form a zygote (VI). VII. Zygote after growth to full size. VIII, IX. Production of swarm spores by the zygote. X. Vegetative colony formed by fissions of the swarm spore.

wrhile in Volvox, where the fully developed colony may include as many as twelve thousand or more individuals, only five to fifty cells, scattered through the posterior half of the colony are gonidial, the remainder being vegetative (Fig. 10).

Among most of these colonial Flagellates, and indeed in many other Protozoan groups, at irregular intervals these gonidial cells lose the property of directly giving rise to new colonies, and become very highly differentiated in structure and in behavior. These highly modified gonidia are termed gametogonidia, the ordinary gonidia being then given the name

Fig. 9. — Colony of the Flagellate, Pleodorina Illinois ensis. Lateral view. After Kofoid. A, anterior; g, gonidial (reproductive) cells, v, vegetative cells.

of parthenogonidia. The gametogonidia form specialized cells termed gametes which must meet and fuse in pairs, i.e., conjugate, before reproduction may proceed. In the simplest cases these two gametes are nearly or quite alike. In most cases, however, gametes of two very unlike forms are produced by different gametogonidia. These must conjugate in pairs of unlikes before reproduction is possible. A series of forms illustrating the progressive differentiation of the gametes, or germ cells, is described in Chapter V. For the present we may merely notice that in such a colonial form as Volvox, under certain conditions, the parthenogonidia cease reproducing; certain of them {oogonidia or ovaries ) enlarge, and each differentiates a large non-motile cell, the ovum, or oosphere, or macrogamete, while others ( spermagonidia , or spermaries ) after enlarging similarly, divide repeatedly, each forming a large number, often as many as one hundred and twenty-eight, small actively motile cells termed sperm cells or microgametes. A single sperm from one colony then meets and conjugates with a single ovum from another colony, forming thus a single

Fig. 10. — Reproduction in the colonial Flagellate, Volvox. A. After Pringsheim, B-E, after Klein and Schenek. A. Young colony showing distinction between, s, somatic cells, and o, reproductive cells (parthcnogonidia). B. Older colony showing, p, parthenogonidia, o, ofigonidia, sp, spermagonidia in various stages of formation. C. Spermagonidium consisting of thirty-two spermagametes, seen from the side in D. E. Spermagametes more highly magnified.

cell or zygote, which through continued fission gives rise to the individuals of a new colony.1

1 Useful diagrams of these forms of reproduction and gamete formation are given in Hegner, “An Introduction to Zoology,” New York, 1910, pp. 112-115.

Reproduction following gamete formation and fusion (syngamy ) is commonly known as “sexual” reproduction, while the term “asexual” is applied to modes of reproduction which do not involve the fusion of gametes. It is now clear, however, that several different and unrelated processes are included under each of these heads. Thus as “asexual” must be included such diverse modes of propagation as budding, fission, brood formation, parthenogenesis, development from spores, and so forth; and “sexual” reproduction would also follow many forms of syngamic fusion. Two further conditions tend to rob these terms of their precise significance; these are, the existence of transitional conditions, some of which have been mentioned above, and the doubtfully essential character of the primary relation of syngamy to reproduction. These useful terms are to be retained only as convenient though inexact expressions, and are to be used in much the same way that we still employ the convenient words “vertebrate” and “invertebrate.” Even so, we may avoid any unintentional implications by substituting the more exact terms amphigony and monogony for sexual and asexual respectively.

Without suggesting the idea of direct relationship among any existing forms, we may say that it is but a short step from the processes of gamete formation and reproduction among the colonial Protozoa, to the mode of reproduction characteristic of the Metazoa, which after all may be considered highly organized cell colonies. One of the more distinctive as well as more obvious characteristics of the Metazoa is the structural and functional differentiation of large groups of cells as tissues, each variety of tissue performing, in the animal economy, chiefly one function, such as conduction, support, or excretion. Among these various tissues is the reproductive or germinal tissue, which, in all save a few of the lower Metazoa, is always in the form of definite organs, the gonads. The distinction, suggested by some of the colonial Protozoa, between the reproductive and the vegetative tissues was probably the earliest of those “physiological divisions of labor” which involved tissue differentiations in the Metazoa. The essential cells of the gonads, which correspond functionally to gametogonidia, divide repeatedly, the products of their multiplication being, with few exceptions, ultimately thrown outside the body as the highly modified germ cells or gametes. In all the Metazoa these germ cells are of two quite unlike forms; the ova or “eggs,” are large and ordinarily non-motile, while the spermatozoa or sperm cells, are small and actively motile. These Metazoan germ cells clearly correspond with the ova or macrogametes and the sperms or microgametes of certain of the Protozoa. The gonads forming the ova and spermatozoa are known as the ovaries and testes respectively. In most of the Metazoa germ cells of only a single kind are formed by a single organism, a condition which leads to the primary distinction of sex; individuals forming ova are called females, those forming spermatozoa, males.

In comparatively few kinds of animals does a single individual normally possess gonads of both types, and thus become capable of forming both kinds of germ cells, either simultaneously or successively. Such a condition is known as hermaphroditism; it occurs chiefly among the lower Metazoa, such as the Platyhelminthes, Nemertea, some Annulates and Tunicates, and less frequently among the Molluscs, Echinoderms, Bryozoa, Brachiopoda, and Crustacea. Among the Chordata normal hermaphroditism is found only rarely (someTeleosts), though it ma) occur as an abnormality in any group. In a few animals a special form of hermaphroditism occurs, where a single gonad may produce first sperm and later ova, a condition known as protandry and found in some Nematode worms and in the Cyclostome, Myxine, for example. The process of forming first ova and later spermatozoa, known as protogony, is very rare among animals.

In the reproduction of all except a very few of the Metazoa, the initial phase is the union of an egg cell and a sperm cell; this process is known as fertilization or syngamy, and the double cc thus formed, which is called the zygote or oosperm, then gives rise directly to the new individual. Not all of the germ cells formed by an organism actually happen to give origin to new organisms, although any may do so. But with infrequent exceptions, where unusual methods of reproduction occur at times, a few of which have been noted above, new Metazoan individuals arise only from the union of two germ cells.

The substance which forms the reproductive cells or gametes of an organism is called the germinal substance, or briefly, the germ. This is visibly distinguishable at a very early period in the existence of the new organism, from that material which is to form all of the remainder of the organism, in distinction known as the somatic tissue or soma , or simply as the body. Among the higher forms these two kinds of substance — germ and soma — have very different histories and fates. According to the theory of Germinal Continuity, elaborated by Weismann, the germ represents or contains an organic substance which has been in a living state since the beginning of life, and which must continue in this state, in some form, as long as living things shall be produced. The soma, on the contrary, is thought to be built up around the germ, anew and under its influence, in each generation of organisms. Upon the death of the individual it is destroyed completely as living substance; somatic cells finally leave no descendants. Thus in species which reproduce by this method the soma or body is wholly temporary, while the germ may properly be said to be potentially ever enduring. For while actually the greater part of the germ substance formed in an organism is destined to perish, either before the body or with it, or at any rate with the race, some germ must always remain, producing the generations of the future. The essentials of this idea are expressed in the accompanying diagram (Fig. 11).

On account of its usefulness the value and significance of the distinction between germ and soma are frequently overemphasized. In many organisms the distinction can scarcely be drawn at all, for under certain conditions, either normal or unusual, cells which are evidently “somatic” may take on reproductive characteristics and function as germ cells. Many such cases are known among animals, and among the plants reproduction from somatic tissues and cells is very

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common, indeed in some it appears to be the normal method of reproduction.

Among the Protozoa this familiar distinction between germ and soma cannot be drawn at all. In these simple forms the

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Fig. 11. — Diagram illustrating the theory of Germinal Continuity. A, B, C, represent successive generations; g , gamete (sperm cell) produced by another organism; 2, zygote. White circles indicate successive cell divisions within the somatic tissues, the existence of which terminates with the organism of the given generation. Solid black indicates the germ. Dotted circles indicate gametes which may perish, or may unite with those of another organism.

process of reproduction is not so directly associated with the processes of gamete formation and fusion. Nearly all noncolonial Protozoa, while in the so-called vegetative state, have the power to reproduce by fission, so that the plasm of these cells is both germinal and somatic in the Metazoan sense. It is only at irregular intervals that the individuals ordinarily multiplying by fission, lose this property as well as their vegetative characteristics, become specialized as gametes, and require to undergo syngamy, later resuming their duplex vegetative and reproductive character.

While it might be misleading to say that the reproductive cells of Metazoa are, like the Protozoa, both germ and soma, yet it is quite true that in these germ cells we have a substance which produces both germinal and somatic tissues. In a sense we are hardly justified in saying that the soma is built up anew in each generation, while only the germ has a continuous existence. The germ cell is potentially soma as well as germ, and for a time during the early development of the organism there is no visible distinction; this distinction occurs very early in the development of a few forms, but in most organisms not until a considerable number of cells has been formed. In development the germ cells give rise to other cells like themselves (germ) and to cells unlike themselves (soma) and we may regard the “unlike” as “new.”

The common conception of the life of a species as a succession of generations of individuals linked together by the germ, while superficially true, leads to a fundamentally erroneous point of view. The fertilized germ cell is just as much the individual organism as the matured individual is. The species is no more a succession of somas than it is a continuous germ. It is not the function of the germ to provide links between successive generations of “organisms” or somas, any more than it is the function of the soma to insure the continuity of the germ, and to provide materials for its increase and means of its dispersal. We should recognize that the essential continuity between successive generations is, after all, not continuity of plasm but “continuity of organization.”

The term reproduction, strictly speaking, does not mean quite the same thing among Metazoa and simple Protozoa. Among the Protozoa the formation of free daughter cells, by fissions of the zygote or its descendants, constitutes reproduction. Among the Metazoa the corresponding fissions of the zygote and its daughter cells are not considered in themselves reproductive processes, but as steps (cell divisions) in the building up of the whole new individual, as but one phase in the general process of reproduction.

Some physiological reorganization of substance, such as ordinarily results from the intermingling of the plasmas of two individuals or lines, seems a necessity for the continued existence and reproductive activity of most organisms. In some Insects (Aphids, etc.), Rotifers, Crustacea, and other forms, reproduction occurs normally, through long periods, without any such syngamic fusion, the new organisms developing from single unfertilized ova ( parthenogenesis ). While this condition is, in these cases, clearly derived from the normal, yet it seems to illustrate the non-essential relation of syngamy and reproduction. In such forms syngamy does occur under certain conditions or during certain periods in the life cycle of the organism. For example, the difficult conditions of winter or drought may be successfully withstood by the organism while in the form of the zygote.

So too in many, perhaps most, Protozoa, reproduction or fission proceeds normally and for long periods without fertilization or conjugation. The process of conjugation is opposed to reproduction and may actually inhibit it for a time. Here it appears frequently to be associated with the onset of conditions unfavorable to the existence of the organism in its vegetative condition.

It seems, therefore, that the processes of fertilization and

reproduction may not be essentially related, and that the

intermingling of the plasmas of two individuals is related

directly to phenomena other than reproduction. Such a

modification of substance as results from fertilization, however,

may be essential to continued existence, and it is certainly true

of most Metazoa that such a plasmic fusion is an organic

necessity. In the simple Protozoa this may be accomplished

at any time by the fusion of two individuals in the form of

gametes. In the Metazoa, however, it is obvious that this

necessary intermingling of substance occurs only when the 2 organisms are in the form of single cells, i.e., gametes. And thus it comes about that in the many-celled animals, reproduction and syngamy are so uniformly associated, and while these processes may not have been related primarily, in some instances are not even at present, yet now they have come to be so related in the vast majority of Metazoa, that fertilization actually appears as the first and most important step in the whole chain of reproductive events.