A textbook of general embryology (1913) 1
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Kellicott WE. A Textbook of General Embryology (1913) Henry Holt and Co., New York.
Kellicott (1913): 1 Ontogeny | 2 The cell and cell division | 3 The germ cells and their formation | 4 Maturation | 5 Fertilization | 6 Cleavage | 7 The germ cells and the processes of differentiation, heredity, and sex determination | 8 The blastxtla, gastrula, and germ layers. Morphogenetic processes
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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 jgroups 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 iJeproduction. 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 of 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 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 Erst 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 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.
Fig. 1. — aimple fission in Amaba vtapertHio. After Doflein. A. Nonnal vegetative form. B. Commencement of fisBioD ("biscuit form"). C. Fission nearly completed; separation of daughter cells, i, cells o( aa Aita. Zoochlortlla.
The simplest and apparently the most primitive mode of reproduction is that known aa fission, characteristic of the singlecelled 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 mvUi'ple fission or brood fomtaiion 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 iDtuBorioii, Holophrya muUifiliit. After Hatechek. A. Normal vegetative individual. B. Cyst containing the products of repeated binary fission: some of the loCsporea are shown leaving the cyat. C. One of the BoOsporeB, enlarged, cr, oontrftctile vacuole; cv, cyst; m, mouth; ma, macronucleus; mi, micronucleua; I. loCspoieB.
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 spondation and the products are known as swarmspores or zoospores. Technically the term sponilation or sporogony {metagarmtic 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 fisaioD (schiiogoDy) in the Sporoioan, Coccidium lehubergi. After Scbaudinn. A. EntrBote of orgamam ("aporoioite") into epithelial cell of host. B. G. Two Btagea in growth. D, Multiple diviBion of nucleus. E. Daughter nuclei Buperficial, cytoplaam still undivided. F. Fisaioii of the superficial cytoplasm surrounding the nuclei, leaving a central undivided moBS. G, Free achiionts ("meroBoites").
Reproduction by analogous processes also called fission, simple or multiple, is only occasional among the many-celled animals. In a few such forms the entu-e 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 off by the other, fonning new individuals smaller than thei 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 ia known to occur in many genera among the Ccelenterates, and less frequently among the Porif era, 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. "
Fig. 4. — Fission in Metazon. A. B. Two stages in the transverse fisdon of the Actioian, Gonactinia prolifera. From Korschelt and Hdder, after Blochmanii and Hilger C. Successive transverse fissions in the PI aty helminth. Microiilomvm Knmre. After L. von Graff. 1, II, III, mark the levels of the successive fissions: > fourth fission also ia indicated, p. p, pharynges.
In the fission of Metazoa usually many of the structures of the parent are simply transferred to the new organisms with comparatively little differentiation of parts anew, out of a visibly undifferentiated condition. In the Protozoa this may 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 of 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 famiUar 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. 6.— Binary fismoii. ia ing species. It occurs amoug the Si°'wiSW»*%ltlr. Poritera, Ooilentemta, Platyhelmin Stage immediately before the thes, Annulata, Bryozoa, and Tuni separation of the daughter cells.
cata (Fig. 6, B). In budding there ia 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 gemmvles (Porifera), orstafohlasts (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.
Fig. 7. — Reproduction by budding. A. In the InfUBorian, BpAefcta. From CslkinB, "Protozoa." N, branched macronucleus extending from the parent cell into each bud. B. In the Tunicate, Doliolum. AEter Nemnaon. Ventral outETOwth of a phorozooid. 6, reproductive buda of various ages; o, germinal knob; «, stalk.
In some of the shelled Rhizopods (Euglypha, Arcella) a combination of the processes of budding and fission occurs. In this process, called bud-fissicm, 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,
la 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 coeno
FiG, 7. — The formation and development of the Htatoblaat, in the fresh water BryozoBn, Cristaldla. A. After Braem. Othera, after Verworn. A. Longitudinal section through the funiculus, ehowing the relations of the atatoblaBt. B-E. Optical sections of stages in the development of the statoblaet. F. Section shoiviDg rudiment of embryo. «, superficial ectoderm; i. inner layer of funiculus (ectoderm); o, outer layer of funioulua (endoderm); e, rudiment of atato blast.
Mum 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 Pleodorina, of the thirty-two cells forming the colony, four anterior cells are purely vegetative, the remaining twenty-eight are gonidial; while 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).
Fig. 8. — Reproduction in the colonial Flagellate. Pandorijia morum. Proia Hertwig, after PringBbeim. J. Free-swimmiDg vegetative colony. II. Daughter colonies [onned by the fisaions of each individual in a colony rimilar 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 (V/). VII. Zygote after growth to full siie. VIII, IX. Production of swarm spores by the lygote. X, Vegetative colony formed by fissions of the swarm spore.
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 dififerentiated in structure and in behavior. These highly modified gonidia are termed gametogonidia, the ordinary gonidia being then given the name of parthenogonidia. The gametogonidia form specialized cells termed garnet which must meet and fuse in pairs, i.e., conjugaie, 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 enlar^ng similarly, divide repeatedly, each forming a large number, often as many as one hundred and twenty-eight, small actively motile cells termed sj>erm cells or microgametea. A single sperm from one colony then meets and conjugates with a single ovum from another colony, forming thus a single cell or zygote, which through continued fission ^ves rise to the individuals of a new colony.^
Fig. 9. — Colony of the Flagellate, Pleodorina Slinoiienais, Lateral view. After Kofoid. A, aoterior; g, gonidial (reproductive) cells, v, vegetative
Fig. 10. — Reproduction in the colonifkl HaKellate, Volcox, A. After Pringsheim. B~E, after Klein and Schenck. A. Youog colony showing dlBtinction between, a, somatic cells, and g. reproductive cells (parthenogonidia). B. Older colony showing, p. parthenogonidia. o, oCgonidia, sp, Bpermagonidia in various stages of formation. C. Spermagonidium consisting of thirty-two spermagametes, seen from the side in D. E. SpermagaSnetes more highly magnified.
' Useful diagrams of these forms of reproduction and gamete forouition are given in Hegner, "An Introduction to Zoology," New York, 1910, pp. 112-115.
Reproduction following gamete formation and fusion (^j/ngamy) 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 dififerentiation 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 germirtal 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 repro- . ductive 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 thiese 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 (some Teleosts) , though it may 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, Myodne, 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 d^ fertilization or syngamy, and the double cell 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 imion of two germ cells. •
The substance which forms the reproductive cells or gametes of an organism is called the germinal svbstance, 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 som^, 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 Uving 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 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 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 imdergo syngamy, later resuming their duplex vegetative and reproductive character.
Fig. 11. — Diagram illustrating the theory of Germinal Continuity. At Bt C, represent successive generations; gf, 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.
While it might be misleading to say that the reproductive cells of Metazoa are, hke 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 fertiUzed 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 Ufe 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 tru« 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 interminghng of substance occurs only when the 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.
The actual processes involved in the formation, from the zygote, of the mature Metazoan individual are extremely complicated and diverse, but they are for the most part reducible to three fundamental general processes. We must leave aside, for the present, the causal or directive processes which, though probably the essentials of development, are still obscure and little known. The grosser external phenomena of development are, essentially, growth, cell division, and dififerentiation. The living germ is contained within the limits of a single cell, often of minute dimensions and only slightly differentiated visibly; the mature organism consists of an enormous number of cells, comprising a considerable mass, and exhibiting various degrees of differentiation in diverse directions. The transition from one of these states to the other is a gradual process, proceeding by minute steps; yet it is convenient to consider the whole life history of an organism as a succession of phases, each with some chief characteristic.
First, complex processes occur within the gametes or germ cells themselves, concerning chiefly their nuclei, as a result of which they come to have a constitution quite unlike that of the somatic nuclei. These preliminary events we group under the term gametogenesis, or oogenesis and spermatogenesis in the ova and sperm cells respectively. Then normally follows fertilization or syngamy, the fusion of the two gametes, derived from two different organisms, into a single cell which is the "new" organism. Through fertilization a typical nucleus is reconstituted in the zygote, and there follows a period of rapid cell multiplication which is called the period of segmentation or cleavage. During cleavage are formed the cellular elements which are to be built into the structures of the simple embryo, and various differentiated substances of the egg are segregated among different groups of cells. Following this are the phases of blastula formation, when the cells become arranged in a definite layer, and then gastrula formation when the cells are rearranged into two definite layers. Then comes the period of embryo formation, when the cells of the layers are moulded into the earUest beginnings of the chief systems and organs, blocking these out in the simplest manner. During this last phase growth becomes very rapid, accompanied by continued cell •division, no longer termed cleavage, and £is the formation of organs becomes more complete and more particular, the embryo increases in bulk and dimensions. This period of embryonic development may occupy a long time, and usually leads to the formation of an organism which is capable of leading an independent hfe, either as a larva or as a form closely resembling the adult, except in size. Finally, accompanied by continued growth, the last phases of development appear as cellular differentiation becomes more complete, and the organism begins to assume more fully the characteristics of its parents. When the reproductive tissues become functional as such, the animal is considered mature and its development complete, although in a true sense development is never entirely completed, for the form of the organism never becomes definitely fixed, and cellular differentiation seems never to cease during the life of the organism.
It is important to remember that all of these phases of development are continuous and more or less overlapping, and in all of them, excepting perhaps the earlier, where the important changes concern chiefly the structure and the composition of the germ nuclei, the three processes — growth, cell division, and differentiation — are going on together. Yet in general it is clear that in the early stages of development, after the gametic nuclei are differentiated and fused, cell division is the process of greatest activity; then follow stages during which development is characterized chiefly by growth; and lastly the final aspects are chiefly the result of cellular or tissue differentiation, processes often described separately under the term histogenesis.
This brief outUne reflects the fundamental character of the relation of Embryology, as of all biological science, to the Cell Theory. The recognition of the ovum (Schwann, 1839; Gegenbaur, 1861) and spermatozoon (Schweigger-Seidel, 1865) as modified cells, of the basic importance of cell continuity in development (Virchow), and of the processes of fertiUzation, cleavage, growth, and differentiation as essentially cell processes, marked noteworthy and fundamental steps in the history of the science of development. But our recognition of the importance of this relation, and of the especial importance of the cell as the descriptive unit in development, should not obscure the fact that in many developmental processes we cannot recognize the cell as the actual unit of physiological activity. Many important steps in development concern elements which are distinct and individual intra-cellular elements. And later, during the cleavage period, the boundaries of specific materials behaving as units in development do not always coincide with cell boundaries or distributions. We must regard the view that the cells are the ultimate units in development as a stage in the history of opinion, and for the present recognize certain intra-cellular elements as the "ultimate" structures in development.
But the province of Embryology is not merely thus to describe the upbuilding and unfolding of the structure and form of the new organism through these successive stages of development; it is, further, to describe the more fundamental processes involved in this development, and still further, to summarize these descriptions of both kinds in the form of simple general statements or laws. In the historical development of the science of Embryology, as of any natural science, the description and comparison of visible forms and conditions came first. This morphological account of development, concerned chiefly with the description of what happens, what is produced in development, has now been accomplished to such an extent as to furnish a basis of this kind sufficient for immediate necessity. Next comes the study of the real processes leading to the production of one condition out of another, processes which underUe the externally visible form changes. This physiological aspect of Embryology is concerned more with how development occurs, how, and through the operation of what factors or mechanisms, one condition leads to another. In a way this is also the why of development — not "why" in the philosophical sense of course, but in the sense of how does it happen that" these things occur in development. Here the two methods of observation and experiment are combined and by the artificial modification or the elimination of one condition of development after another, the essential factors are discovered and their modes of operation determined. The science of Embryology has now fairly entered upon this stage and the dominant note of the subject to-day is this search for underlying processes and modes of action. But as yet it is impossible to say that we have reached the final period of the formulation of the broad fundamental generalizations which give unity to the infinitely diverse phenomena of development, and which are expressed in the form of laws. While something has been accomplished in this direction, the basis of fact is not as yet sufficiently broad, and the necessity of frequent restatement of such "laws" shows their formulations to be premature, save as guides in investigation.
These steps in the development of the science of Embryology do not so nearly represent the course of thought and hypothesis as that of actual knowledge and achievement. For even in the eighteenth century the earliest embryologists had their hypotheses as to the causes of this mysterious process of development. They offered first what seemed to them an explanation of the facts of development which came to be termed the idea of "evolution" or preformation. This idea was that within the germ, either in the egg ("ovists") or in the spermatozoon . ("spermists, " "animalculists") there was contained a miniature organism resembling, in a general, or even in a precise way, the adult form (Fig. 12). And this miniature had merely to expand, or to unfold and grow, to produce the individual of the next generation. The relation between the germ and the adult seemed much like that between the bud and the branch — all the parts present in minute rudiments, ready to come forth and expand. We may recognize in this idea a morphological conception of development such as we should expect to appear first. This conception, with which are associated such great names as Malpighi, Bonnet, and Haller, proves in reality an attempt to explain development by denying its occurrence. For the assumed formation of the original individuals of a species by the Creator involved at the same time the creation, within them, of the preformed germs of all the other later individuals of the species. The belief that the germ cells of an organism contained in miniature the members of the second generation necessitated the further belief that in these latter must be contained, within still smaller Umits, the individuals of the third generation, and thus ad infinitum. And so it was estimated that some two hundred millions of human beings were actually contained in/'of ' a"humr; in this preformed condition within the ovaries Bperm cell contain- q£ ^ rpy^ conception of infinite encase ing a miniature oiv ^
ganism enclosed in ment OT ^^ewbditermnV^ proved to be the
a thin membrane. i ,• j x. i i?Jii-L £
After o. Hertwig, TediLctio od dosurdum of the theory of pre^.l^J!^.. Hartsoeker formation in this its first and crudest form.
(1694).
Those who actually observed the chick appear within the egg could not accept this naive explanation of development, but believed that there occurred a true formation of parts anew out of unformed material not possessing at all the characters of the adult organism. This was Wolff's idea of epigenesis, clearly a physiological conception of development, following quite naturally the earlier morphological conception. In its original form epigenesis was chiefly a dissent from the idea of preformation rather than an explanation of development. Indeed it seems now to have been merely a restatement of the fact that development occurs, leaving this fact to be explained through the operation of some supernatural or miraculous process, for the spontaneous generation of the embryo within the egg was at j&rst definitely assumed.
Thus we have almost from the beginning of embryological study, two opposing explanations of the visible phenomena of development, preformation explaining development by denying it, epigenesis explaining development by reaflBirming it. Since this early conflict of opinions, the crudity of which we understand when we think of the means then at hand for observing such minute objects as are many eggs and embryos, there Has been constant opposition of morphological and physiological interpretations of development. The modem understanding of preformation is better termed predelineationj or better stilly predetermination, less crude, less complete and particular than preformation. What is preformed or predetermined in the germ in some way represents the embryo without being at all Uke it. The idea of epigenesis, too, is to-day less complete; a certain structural organization is admittedly present in the germ as a heritage from previous generations, and real development occurs as a physiological process directed by this rudimentary structure already present. The history of thes6 opinions indicates that neither conception is exclusively true, but that development must involve both predetermination and epigenesis; and the present endeavor is to find out not which, but to what extent each, is true.
The present understanding of development seems to be an extremely refined predetermination strongly tinged with epigenesis, using these words in their modem sense. A more extended statement of this modern view and the facts upon which it is based is reserved for a later chapter (Chapter VII). Briefly stated, we believe that while the embryo, not to say adult, is by no means preformed nor even fully predelineated in the germ, yet there is a certain degree of protoplasmic structure or regional differentiation in the germ cells. This is spoken of now as the organization of the germ, and it may be both material and dynamic {i.e., energetic). And further this organization is definitely related to the structure of the future embryo and adult, having reference, but not resemblance, to the adult. The organization of the cytoplasmic part of the germ is itself a condition which develops (epigenesis) under the influencie of the primary structure, or organization, of the nucleus. At present this inherited organization or predelineation of the nucleus seems primary and fixed, and to represent the only strictly predelineated portion of the germ, controlUng and directing the later and epigenetic developmental processes, which may be said often to have commenced in the germ cells even before syngamy has occurred. But history warns us against believing that this organization of the nucleus will prove the ultimate organization* As knowledge becomes more complete this will be thrown farther back to restricted elements of the nucleus; indeed it seems probable now that the primary organization concerns, not the entire nucleus nor perhaps its chromosomal elements alone, but some, as yet invisible, problematic, chemical and physical configurations of its structure.
But we must not search for an explanation of the whole process of development, alone in the structure of the germ cells. We must look upon development as upon other forms of activity in living things, as a succession of reactions on the part of the organism to the normal stimuli of its surroundings. The things that an adult organism does are obviously reactions; it reacts to the conditions of its environment by making certain movements, forming certain substances, undergoing certain structural modifications; in short, by doing certain things collectively termed its behavior. The precise character of an animal's behavior is determined not alone by its structure, by the organs it has to react with, nor alone by its physiological condition at the time, nor alone by the nature of the external conditions acting; but by all of these combined. What the adult organism does at any particular moment is therefore determined by two interacting sets of conditions, one within the organism — ^its organization, the other without the organism
BARTON C *^ RSSSLP^ — ^its environment. Either set of conditions alone can lead to no action; for organismal activity is reaction.
Just so the developing organism, at whatever stage it be considered, reacts to the stimuli of its environment in a manner determined for the moment, on the one hand by its own state or "organization," both morphological and physiological, and on the other by the character of the stimuli acting. The ovum is not to be regarded as a mechanism wound up, ready upon receipt of a single stimulus, to go through its development into an adult organism. It is rather to be regarded as an organism which reacts to its surroundings by undergoing certain changes. This changed organism then reacts further by undergoing certain other changes. One reaction of the fertilized ovum is to cleave, of the blastula to gastrulate, and so on. Step by step, one condition succeeding another and leading to still another, the organism gradually alters its morphological and physiological characteristics. Throughout its whole existence the organism shows transformations of substance, energy, and form; we agree to set apart certain of these transformations occurring at a very early period, and to refer to them as processes of " development.*' The normal "behavior" of the egg or of the embryo is to develop. The processes of development are neither easier nor more difficult to explain than the phenomena of adult behavior, and they have just the same basis in the relation between the internal conditions, within the organism, and the external conditions, without the organism, at the time.
From this point of view the question why the egg develops is a problem not different, in its essentials, from why the organism grows, or why it seeks or avoids the light. None of these is to be solved by consideration of the organism alone, whether egg or adult, apart from the conditions acting upon the organism; both must be studied together.
With this conception of development in mind, we should here mention briefly one of the great generalizations that has come from the study of organic development, namely, the Biogenetic Law or the Theory of Recapitulation. Briefly stated this familiar theory is, that the organism, in its individual developmental history, tends to repeat in outline the evolutionary history of its species. This repetition is seldom particular, or detailed, never complete, yet so many of the phenomena of development can be satisfactorily interpreted from this historical point of view, seeming to have this historical significance rather than an immediately adaptive relation, that as a general statement the law remains fundamentally true.
This law is not so much an attempt to resume the facts of embryology as to apply these facts in the interpretation of racial history (Evolution). This application is in many instances difficult because of the fact that there has been an evolution of the egg, the embryo, and the larva, just as of the adult. The fact that the organism is specific at all stages of its existence, includes the parallel evolution of ova, and of all succeeding
Species
Zygote
Stages in Development
Adult
D
Aj>
Bj, Cj,
D
E Ag B, Cm Dm E
F A, B, C, Dr E, F
G Ao Bo Co Do Eo Fo G
H An Bu Cu Dg Eg F„ (?„ H
Fig. 13. — Diagram to illustrate the essentials of the Biogenetic Law. Modified
from O. Hertwig.
developmental stages, if there is to be any evolution of adult structures, else diversity of adult organization would depend upon external conditions of development, rather than upon egg organization. But we know that the eggs of any species of sea-urchin and star-fish will develop, respectively, into adult sea-urchins and star-fish of those species, although in the same dish, with identical environing stimuli.
Many important points concerning the relation between ontogeny and phylogeny may be represented schematically, as in the accompanying diagram (Fig. 13). Here we compare the ontogenies of five related species, the adults of which represent an evolutionary series; species E has evolved beyond D, F beyond E, and so forth. The first stage (i.e., the fertilized ovum or zygote) of D is not merely a zygote (A), but it is the zygote of species D, and consequently indicated in our diagram by A^; in its development this passes through the specific stages Bd, and Cp, to the adult D. Species E is more highly evolved than species D, but it begins its existence as a fertilized ovum which again is specific, this time Ajj. In its development to the adult form this may pass through stages B and C similar to those of species D, but merely similar, not identical, else the result would be D and not E. Therefore, we call these intermediate stages B^ and C^. Further, species E may pass through a stage in some particulars resembling D; this, however, does not exactly resemble D and is therefore designated D^. Similarly for species F, G, and H; each is more highly evolved than the preceding. Each passes through stages which resemble stages in the development of the less highly evolved species, yet each stage is really specific.
Conditions of life change for the embryo as well as for the adult, and if these younger organisms are to remain in existence they must evolve to meet the changed conditions. The process of evolution concerns not merely the adult, but the organism at every stage of its existence. Stages such as Bq or C^ may finally become so highly modified that they are no longer recog- • nizable as related to B and C, and might as well be termed Xq and Yfl. It is then said that these traits are "ccenogenetio modifications" in distinction from "palingenetic characteristics," which are obvious similarities to previous racial conditions. But recognition of the idea that the entire life history is undergoing evolution, at every point, very largely minimizes the value of this very common distinction between coenogenetic and palingenetic traits in development, for in a very true sense all the traits of the developing organismjs are in varying degrees both coenogenetic and pahngenetic.
Finally, we see that the problem why the egg develops into a form resembling its progenitors, rather than organisms of another kind, that is to say, the problem of heredity, may be more clearly understood by recognizing that the characteristics of the organism are specific at all stages of its existence. The egg of the star-fish is just as much a star-fish 05 the adult is. The germinal substance of successive generations of star-fish is directly continuous. This continuity of specific organization through the germ, combined with essentially uniform conditions of development, determines the essential uniformity of each series of interactions leading to the formation of a new adult organism. In a real sense the problem of heredity thus becomes the same as the problem of development. And the problem why the egg of the star-fish develops into a star-fish and not into a sea-urchin, is fundamentally the same as the problem why the star-fish is not a sea-urchin; it is the general problem of the evolution of organic diversity.
References to Literature
In the "references to literature," given at the end of each chapter, the author's name and the title of the work, are followed by the reference to the journal in which the work appeared, or to the place of publication, in case the work is a separate pubhcation. The number of the volume (Band, tome, etc,) is printed in black-face Arabic numerals, followed by the year of appearance. References to pages, parts, etc., are omitted except in a few necessary instances.
The abbreviations of the more common references are as follows: Amer. Jour. Anat. American Journal of Anatomy. Baltimore and
Philadelphia. Amer. Jour. Physiol. American Journal of Physiology. Boston. Amer. Nat. American Naturalist, Boston and New York. Anat. Anz. Anatomischer Anzeiger. Jena. Anat. Hefte. Anatomische Hefte. Wiesbaden.
Arch. Anat. u. Entw. Archiv fiir Anatomie und Entvdcklungsgeschichte. Arch. Anat. u. Phys. Archiv fiir Anatomie und Physiologic. Leipzig. Arch. Biol. Archives de Biologic. Leipzig and Paris. Arch. Entw.-Mech. Archiv fiir Entwickelungsmechanik der Organismen.
Leipzig. Arch, gesamte Physiol. Archiv fiir die gesamte Physiologic des Menschen und der Tiere. Bonn. Arch. mikr. Anat. Archiv fur mikroscopische Anatomie und Entwicke lungsgeschichte. Bonn.
Arch. Protist. ArchivfUr Protistenkunde. Jena.
Arch. Zellf. ArchivfUr Zellforschung. Leipzig.
Arch. Zool. Exp. Archives de zoologie experimerUale et gSrUraL Paris.
Biochem. Bull. Biochemical BvMetin. New York.
Biol. Bull. Biological Bulletin. Woods Hole, Mass.
Biol. Centr. Biologisches Centralhlait. Leipzig.
Botan. Gaz. Botanical Gazette. Chicago.
Bull. Mus. Comp. Zool. Harvard Coll. Bulletin of the Museum of Com- paraiive Zoology al Harvard College. Cambridge, Mass. Ergebnisse und Fortschr. Zool. Ergebnisse und Fortschritte der Zoologie.
Jena. Ergebnisse Anat. u. Entw. Ergebnisse der Anatomie und ErUvdckelungs- geschichte. Wiesbaden. Jena. Zeit. Jenaische Zeitschrift filr Naturwissenschaft. Jena. Jour. Anat. Phys. Paris. Journal de Vanatomie et de la physiologie
normales et paihologiques de Vhomme et des animaux, Paris. Jour. Coll. Sci. Imp. Univ. Tokyo. Journal of the College of Science,
Imperial University of Tokyo, Jour. Exp. Zool. Journal of Experimental Zoology. Baltimore and
Philadelphia. Jour. Morph. Journal of Morphology. Boston and Philadelphia. Mitt. Stat. Neapel. MitteUungen aus der zoologischen Station zu Neapel.
Berlin. Morph. Jahrb. Morphologisch£s Jahrbuch. Leipzig. Phil. Trans. Roy. Soc. Philosophical Transactions of the Royal Society
of London. Pop. Sci. Mo. Popular Science Monthly. New York. Proc. Am. Phil. Soc. Proceedings of the American Philosophical Society.
Philadelphia. Q. J. M. S. Quarterly Journal of Microscopical Science. London. Sitz.-Ber. Acad. Wiss. Berlin. Sitzungsberichte der koniglich preussis^
chen Akademie der Wissenschaften zu Berlin. Sitz.-Ber. Phys.-Med. Ges. Wtirzburg. Sitzungsberichte der Physicalisch- msdizinisch GeseUschaft zu Wurzburg. Sitz.-Ber. Ges. Morph. Phys. Sitzungsberichte der GeseUschaft filr
Morphologie und Physiologie in MUnchen. Trans. Am. Phil. Soc. Transactions of the American Philosophical
Society. Philadelphia. Zeit. Indukt. Abstamm. Vererbungslehre. Zeitschrift filr induktiva
Abstammungs- und Vererbungslehre. Berlin. Zeit. wiss. Zool. Zeitschrift fiir wissenschafUiche Zoologie. Leipzig. Zool. Jahrb. Zoologische Jahrbiicher. (Abteilung ftir Anatomie und
Ontogenie der Tiere, imless otherwise specified.) Jena.
REFERENCES TO LITERATURE — Chapter I
Calkins, G. N., The Protozoa. Columbia Univ. Biol. Ser. VI. New York. 1901. Protozoology. New York. 1909.
DoFLEiN, F., Lehrbuch der Protozoenkunde. Jena. (3 Aufl.) 1911.
Hartsoeker, N., Essay de dioptrique. Paris. 1694.
Hertwiq, O., Zeit- und Streitfragen der Biologie. I. PrfifOrmation oder Epigenese. Grundztige einer Entwickelimgstheorie der Organismen. Jena. 1894. English translation by P. C. Mitchell, "The Biological Problem of To-day: Preformation or Epigenesis," etc, London. 1896. Die Entwickelungslehre im 16. bis 18. Jahrhimdert. Handbuch der vergl. u. exp. Entwick. d. Wirbelt. I, 1, 1. Jena. 1906. (1901.) Ueber die Stellung der veigleichenden Entwickelungslehre zur vergleichenden Anatomie, zur S3rstematik und Descendenztheorie. (Das biogenetische Grundgesetz, Palingenese imd Cenogenese.) Id. Ill, 3. 1906.
Hertwig, R.; Mit welchem Rechte unterscheidet man geschlechtliche und ungeschlechtliche Fortpflanzung? Sitz.-Ber. Ges. Morph. Phys. 16. 1899. English translation by Curtis, W. C, Science. 12. 1900.
KoFoiD, C. A., On Pleodorina illinoisenMs, a new Species from the Plankton of the Illinois River. Bull. 111. State Lab. Nat. Hist. 5. 1898.
KoRSCHELT und Heider, Lehrbuch der vergl. Entwickelungsgeschichte der wirbellosen Thiere. IV Abschnitt. Ungeschlechtliche Fortpflanzung and Regeneration. Jena. 1910.
LiLLiE, F. R., The Theory of Individual Development. Pop. Sci. Mo. 76. 1909.
LocY, W. A., Biology and its Makers. New York. 1908.
Morgan, T. H., The Problem of Development. International Monthly. Burlington, Vt. 1901.
ScHULTZ, E., Prinzipien der rationellen vergleichenden Embryologie. Leipzig. 1910.
Weismann, a., Essays on Heredity. English Translation. I and II Series. Oxford. 1891, 1892. The Germ Plasm. English Translation. New York. 1893. The Evolution Theory. English Translation. New York. 1904.
Whitman, C. O., The Inadequacy of the Cell-theory of Development. Woods HoU Biol. Lect. 1893. Bonnet's Theory of Evolution. Id. 1894. Evolution and Epigenesis. Id. 1894.
Wilson, E. B., The Problem of Development. Science. 21. 1905.
