Text-Book of Embryology 2-9 (1919)
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Kerr JG. Text-Book of Embryology II (1919) MacMillan and Co., London.
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Chapter IX Some of the General Considerations Relating to the Embryology of the Vertebrata
IN the course of the preceding chapters many of the general principles of vertebrate embryology will have made themselves apparent: the present chapter will deal shortly with some others of these principles which seem to require special notice.
1. The Ontogenetic Evolution Or The Zygote Into The Completely Formed Individual
The vertebrate commences its individual existence as a zygote— a single ce1l——in which the specific characteristics, derived from the paternal and maternal ancestors, are already present though not recognizable. That this latter statement is accurate is demonstrated by such a fact as the following. The pelagic fertilized eggs of different species of Teleostean fishes show no trace of the specific features which characterize the adults. Such distinguishing features as are present and enable a specialist to identify them are mere differences in size, amount of yolk, colour of oil globule and so on, and have nothing to do with adult characteristics. Yet if a selection of such eggs are allowed to develop together under a homogeneous set of environmental conditions each is found gradually to unfold the complete array of characteristics which distinguish its own kind. As the various zygotes have developed under the same identical set of environmental conditions it follows that the differences which gradually become apparent cannot be due to the moulding influence of external conditions: they must have been already present though in invisible form in the zygote.
It follows further that the evolution of the zygote into the adult is in the main not a process of acquiring greater and greater complexity, in the sense of acquiring new properties, but rather of the loca1iza_tion—the segregation—of special peculiarities in particular portions of the individual, so that these portions assume a, specific character and become recognizable as definite tissues or organs. The peculiarities were there to begin with, but they were diffuse and therefore unrecognizable—somewhat in the same way as the various colours of the spectrum are present in ordinary “ white” light but are invisible until they are sifted out from one another by the action of a prism.
The lesson learned from the developing pelagic zygote——that in its case the full equipment of the complete individual is provided from internal sources——-is one which should ever be borne in mind. It makes it- easier to realize that in other cases, where the developing organism exists in a less homogeneous environment and where it has to fend for itself, characters impressed upon it directly by the environment, however conspicuous, are still superficial as compared with the really fundamental characters already present in the zygote.
The course of ontogenetic development from the zygote stage involves two main processes, (1) increase in bulk accompanied by the assumption of a multicellular condition, and (2) differentiation of parts 73.6. the segregation, into localized portions of the living substance, of peculiarities which were in the zygote distributed without definite arrangement. The topographical differentiation of the developing embryo does not necessarily keep exact pace with the subdivision into cells. Thus in Amplviomus the egg appears to be still homogeneous throughout up to the time when it has segmented into 8 or even 16 blastomeres for even at this stage a blastomere isolated from its neighbours experimentally may go on for a time pursuing the same course of development as it would have done were it a complete zygote. In other cases, as appears to be the rule in the Frog, the first step in segregation—-the segregation of characters belonging to the right and left halves of the body into corresponding hemispheres of the egg—-would appear to take place in the zygote stage 7I.e. before the appearance of the first cleavage furrow.
The progressive segregation of specific characters in the various parts of the developing individual is beautifully brought out in the case of various invertebrates by the elaborate studies on “ Cell lineage,” some of which have been fully described in Vol. I.
The animal individual lives its life under a particular set of environmental conditions, constituted by the external medium—— water or air—with its other living inhabitants: the latter play an important part, it may be by such comparatively simple and direct methods as by affecting the composition of the external medium, it may be by far more complex and obscure influences due to biological inter-relationships. The individual is able to go on living because of its organization and its living activities being fitted, in the most intimate manner, into the particular set of conditions which constitute its environment.
So also with the various parts of the body—organs, tissues, individual cells —of the young developing individual. Each lives amidst an environment of extreme complexity and of perfectly definite type of complexity, conditioned by the nature of the body of which it forms a. portion and by the character of the parts of the 486 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.
body which are in proximity to it. Its living substance is everywhere bathed by——and no doubt intimately adapted to life in--an internal medium, watery fluid laden with the products of metabolism of the living substance as a whole. The differences in function of the various organs and tissues necessarily involve difi“erences in their metabolic activity and therefore differences in the chemical nature of the contributions which they make to the complexity of the internal medium as Well as differences in the substances which they withdraw from the internal medium for their own needs.
Physiologists recognize that changes in the constitution of the internal medium play an important part in exciting and controlling vital activity.‘ In the ordinary life of the animal important examples of such influence are aiibrded by changes in the activity of the normal function of an organ, as for instance when the pancreas secretes actively in response to the presence in the internal medium of a special substance secreted by the intestinal wall when stimulated by food material. Other examples are afibrded by changes in growthaetivity-—as of the skin in response to a change in the amount of substance secreted by the thyroid, or of the mammary gland in response to the presence of substances produced by the metabolic activity of the foetus.
There is no reason to doubt that the living cells and tissues and organs of the embryo are similarly adapted to and influenced by the constitution of the internal medium, and if this be so the influence in question must play an important part i11 development. A possible example is aflbrded by the experimental result that ‘the grafting of the developing optic cup of an Amphibian embryo into near proximity to ectodermal tissue (such as the pigment-layer of the retina, the wall of the brain, the olfactory epithelium, the external ectoderm of the head or trunk) is apt to induce that ectoderm to develop into a lens (Bell, 1907). Such influence upon one portion of embryonic substance by another portion in its neighbourhood may well be exercised through chemical or other changes produced by the specific metabolism of the latter in the internal Inedium in its neighbourhood.
A corollary to the consideration outlined above, which has an important bearing upon much work in experimental embryology, is that it is unwise to place reliance upon the mode of development of an organ-rudiment being normal, unless its environment is normal.
2. Cellularization of the Zygote: Cellular Continuity and DiscontinuitY
Protoplasm being a soft semi-fluid substance a particle of it, as it increases in volume during the process of growth which is associated with normal metabolism, would soon reach a mechanically unstable condition, in which retention of its characteristic form, or even cohesion, would be impossible. In nature a corrective to this is provided by the protoplasm undergoing fission. In the Protozoa the products of this fission normally break apart and lead an independent existence, while in the Metazoa the subdivision is less complete and the growing mass of living substance continues to exist as a coherent individual. The physiological advantages of subdivision of the individual body into cellular units is apparent. It renders possible the intercellular deposit of rigid skeletal materials which act as a support to the organism as a whole: it facilitates localization of function and enables blocks of units specialized for particular functions to be transferred d11ring ontogeny to the positions in which they will be most useful: it enables other units to move hither and thither, either by their own activity or by being swept along in a circulating stream of fluid, to wherever they are specially needed in the course of the ordinary vital processes: and it is of enormous importance in relation to attacks upon the organism from without, whether by limiting the area of injury to comparatively small tracts of living substance or by enabling portions of the living substance specialized for defence to be mobilized and ready to concentrate at the point of attack.
- For their bearing upon evolutionary change see Parker (1909).
Modern science impresses upon us the. importance of regarding the individual not merely as an aggregation of cells and organs, but rather as a mass of living substance imperfectly subdivided up into cells and organs: imperfectly because each cell and each organ is inextricably linked up in the living activity of the whole individual. It brings to our notice numerous tissues in which the actual living substance of the constituent cells is linked up by intercellular bridges of protoplasm: it tells us of particular cases of developing embryos where similar intercellular continuity is apparent. The question is thus raised: are we correct in our belief that actual complete separation of cells takes place as a general rule when they undergo fission during ontogeny? More especially is it really the case that the individual blastomeres of the segmenting egg become completely separated from one another: is it not rather the case that the apparently complete separation is only apparent, that the individual blastomeres remain continuous through fine protoplasmic bridges: and that cases of intercellular continuity observed in the adult are merely expressions of the fact that such bridges persist throughout the whole period of development ?
That the latter is really the case has been held by various workers and supported particularly strongly by Sedgwick (1895, 1896). It will however have been gathered by the reader from Chapter I. that such a view is in the opinion of the present writer not tenable. The fact that the blastomeres of a segmenting egg tend to take a spherical form, or at least to be bounded by convex surfaces, seems by the ordinary laws of surface tension to indicate that these blastomeres are not continuous with one another. Continuity of substance between the cells of the embryo or adult is therefore when it occurs a secondary and not a primary phenomenon. At the same time the present writer’s observations lead him to agree with Sedgwick that such intercellular continuity ‘of protoplasm is much more widely spread than is generally recognized.
3. Yolk
Theoretically the most primitive type of zygote should from the beginning be able to absorb food for itself. As an actual fact however the zygote is provisioned for a shorter or longer period by the highly nutritious fat and proteid, in the form of yolk which is stored up in its cytoplasm.‘ With increasing specialization the amount of this store becomes greater and greater so as to lengthen the period during which the young individual is provisioned and freed from the necessity of working for its own living. A good example of a high degree of such specialization amongst Vertebrates is afforded by the relatively huge egg of the Ostrich.
It has of course to be borne in mind that the degree of specialization in this direction is to be estimated not merely by the absolute amount of yolk present but still more by the relative amount of yolk in proportion to protoplasm. Thus two eggs may be described as equally richly yolked although very different in size provided that the proportion of yolk to protoplasm is similar in the two cases. In correlation with this we find that a group characterized by heavily yolked eggs may evolve in the direction of producing more and more numerous, and therefore necessarily smaller, eggs. Good examples of this are seen in the Teleostean fishes where the eggs may be produced in enormous numbers and of very minute size although still retaining a proportionately large supply of yolk.
In C. Rabl’s discussions of his “ Theory of the Mesoderm ” (1889) an important place is taken by repeated losses and re-acquisitions of yolk during the phylogeny of the Vertebrata. Rabl arranges Cyclostomes, Elasmobranchs, Ganoids, Amphibians, meroblastic “ Prota1nniota” and Mammals, in a linear series, and concludes that Ganoids and Amphibians have undergone a diminution of yolk and have therefore reverted to the holoblastic condition; that meroblastic Protamniota have re-acquired a large amount of yolk and have therefore reverted to the meroblastic condition; and that finally Mammals have lost their yolk and again become holoblastic. In the opinion of the writer there is no sufiicient justification for any one of these assumptions except the last. There is, as is well known, definite evidence to show that Mammals are descended from ancestors with large and heavily yolked eggs and that the small size and practically yolkless character of their holoblastic eggs are secondary acquirements. In this case the loss of yolk has brought in its train profound changes in the early processes of development but of such there are no signs in those other cases in which llabl supposes loss of yolk to have taken place. It must also be remembered that in the Mammal there is an obvious physiological reason for the loss of yolk——namely that the food material needed during the development of the embryo is provided from the tissues of the mother.
- 1 For a detailed account of the development of the yolk in the egg of one of the lower Vertebrates (Proteus) see J orgensen (1910).
On the recapitulation hypothesis the segmentation and other early stages of ontogeny represent ancestral evolutionary stages common to all Vertebrates. The differences to be observed between such stages in different members of the group are consequently not to be looked on as ancestral but rather as due to the influence of disturbing secondary factors. Of these by far the most important is the presence of the particles of yolk, this dead substance clogging and retarding the living activity of the egg protoplasm. 'l‘he extent to which it does this in any particular region of the egg is roughly proportional to its relative amount as compared with the living protoplasm in that portion of the egg. The yolk is as a rule of higher specific gravity than the protoplasm. Correlated with this it tends to be in proportionally greater amount in the lower parts of the egg than in the apical part, with the result that the processes of cell division and of development generally are relatively more slowed down in these lower portions———in extreme cases brought to a full stop——-by its retarding influence. Typical examples of this are seen in the holoblastic but unequally segmenting eggs of the ordinary Amphibia. In this case it is possible by replacing experimenta‘lly the action of gravity by a more potent force (by centrifugalizing the eggs) to concentrate the yolk still more than is natural iii the lower hemi_sphere with the result that the egg is now converted into a meroblastic one (0. Hertwig) the lower hemisphere being unable to segment. On the other hand by inverting the egg and so allowing the yolk granules to settle down towards the apical pole under the influence of gravity it is possible to cause the segmentation furrows to start from the abapical pole and spread towards the apical.
The influence of yolk upon the gastrulation process will have been realized from the perusal of Chapter II.: it is well illustrated by the series Amplmlowus (Fig. 18), Petromg/zon (Fig. 23), Rama (Fig. 25), Lepirlosiren (Fig. 21), II;/pogeoplzxis (Fig. 27) and Torpedo (Fig. 28). Put in a single sentence it may be said to consist above all in the gradual subordination of the process of invagination to those of overgrowth and delamination. In the succeeding stage it makes itself apparent more particularly in the modification of the mode of origin of the mesoderm, the outgrowth of hollow enterocoelie pouches being replaced by the delamination of a solid mass or sheet.
The storage of yolk carries with it not merely the modifications just indicated in the processes of segmentation and_ gastrulation. Its influence becomes retrospective and affects even preceding stages during the growth of the intra-ovarian egg. This is shown more especially by the precocious concentration of yolk in that portion of the egg which will later become endodermal. Thus is the telolecithal condition brought about and telolecithality itself is seen to be really a foreshadowing of a particular adaptive feature of later stages of development (p. 183).
In examining sections of later stages of Vertebrate embryos in which the eggs are rich in yolk it is readily seen that there are conspicuous differences between different parts of the e1nbryo’s tissues in regard to the yolk contained in their cells, for example endodermal structures are frequently marked out by larger yolk granules which cause them to stain more deeply with yolk-staining dyes. The condition of the yolk in a tissue may indeed give a useful hint as to the cell layer to which it belongs and as a matter of fact evidence of this kind has played a conspicuous part in many embryological discussions. '
It is important to bear in mind however the physiological significance of the character in question. It appears to be closely related to the metabolic processes in the tissue concerned. As a given tissue in a yolky embryo goes on with its growth and development its yolk is gradually used up, a necessary preliminary being its breaking down into fine particles easily assimilable. Tissues or cells undergoing active growth and multiplication have their yolk in this fine-grained condition: those which are for the time being comparatively inert retain their yolk in a coarse-grained for“m. Thus a disturbing factor is introduced which has to be carefully borne in mind when using the character of the yolk as a criterion of the morphological nature of a given cell or tissue.
A still further disturbing factor lies in the fact that while yolk is being used up and disappearing from view in one part of the body it may be deposited in cells elsewhere-—as for example takes place in eggs during their period of growth within the ovary. Such increase in the amount of yolk however, accompanied commonly by increase in the size of the individual granules, is naturally relatively rare in comparison with the breaking down of yolk which is occurring through the general tissues of the embryo. It follows that on the whole coarsely granular yolk in a cell or tissue affords more reliable evidence as to its nature than does fine-grained yolk———which may be and usually is merely a symptom of active metabolism.
4. Recapitulation
T fascination as well as the philosophical interest of the study of Vertebrate embryology rests in great part upon the recapitulation of phylogenetic evolution during the development of the individual. In the early days of evolutionary embryology this idea was accepted in an unquestioning and uncritical spirit and it was supposed that all that had to be done to obtain a11 accurate‘ and fairly complete picture of the phylogenetic history of any particular animal was simply to work out its ontogenetic development. The more extensive knowledge which we have regarding embryological phenomena to-day serves on the one hand to confirm fully the truth of the general principle and on the other hand to indicate how its working is interfered with by various disturbing factors.
The main controlling factor in ontogeny is the character of the adult. This is the motive power throughout the developmental period. Just as according to N ewton’s First Law a moving body tends to continue in Ia state of uniform motion in a straight line, so in ontogeny the developing individual tends to progress constantly towards the goal of adult structure. Not in this case however necessarily by the straightest and shortest path. The structure of the adult is the expression of the action of Heredity. The earlier stages are not exempt from the same influence. Each step in the development of the ancestor tends to be repeated in the development of the descendant. The descendant then during its ontogeny tends to pursue the same, it may be devious, path as the ancestor. If in the course of generations the adult structure becomes shifted onwards in a process of evolution, this merely means the adding on of a new portion at the latter end of the ontogenetic path. The earlier portions of this path, built up of similar increments representing previous steps in evolutionary progress, are repeated as before, and so the complete process of individual development forms a record or recapitulation of phylogenetic history.
It cannot be too constantly borne in mind that the factor just indicated is the supreme factor in ontogenetic development. Other factors may be superficially conspicuous, may have far-reaching influence upon details, but this factor—-the tendency to repeat ancestral steps in development up to and including the final characters "of the adult—is and must always be paramount.
Modern advances in knowledge of the facts of embryology, together with the assumption of a properly critical frame of mind, have shown, however, that the picture of past evolution afforded by the phenomena of individual development is at the best but a blurred and imperfect one, and that this must necessarily be so is readily realized when we remember that a large proportion of the characters of any organism are adaptive to its special mode of life. The circumstances under which a developing organism exists are, as a rule, widely different from those under which its ancestors proceeded along the evolutionary path, and in correlation with this its adaptive features are equally distinct. As we study the development of any species of animal we do not then see before us a complete and perfect picture of its evolutionary history, but merely gain fleeting, and it may be misleading, glimpses through the obscuring_ clouds of adaptive features.
A further disturbing factor is indicated by the consideration that in past evolutionary history each stage in evolution was represented by a complete functional organism, all the parts of which were necessarily at correlated stages of development so as to form a functional whole. Many modern animals however develop under conditions in which the different systems of organs are no longer forced to keep accurate step with one another, and the result is that some lag behind while others, particularly organs of great histological complexity in the adu1t——-such as the brain or the eye——are accelerated in their early development, so as to give time for the complicated histogenetic processes that have to be completed before the organ can become functional. It will be realized that this latter type of 492 EMBRYOLOGY OF THE LOWER VERTEBRATES CH.
disturbance affects the development of the individual as a whole much more than it does its component organs, the result being that embryology frequently affords a much more perfect picture of the evolution of single organs than it does of the organism as a whole.
In reference to the ontogenetic record of phylogeny an interesting question presents itself regarding the reliability or otherwise of the information derived from the study of larval forms. To what extent may a particular type of larva be taken as probably representing a corresponding phase in the evolutionary history of the group: to what extent are its features to be regarded as ancestral, to what extent as mere modern adaptations to the environmental conditions among which the particular creature now pursues its individual development? In connexion with various groups among the Invertebrata larval forms have played a conspicuous part in phylogenetic speculation-—in some cases without due discrimination in interpreting their features as ancestral—the climax perhaps being reached by the view which regards such pelagic larvae as trochospheres or naup1ii——precocious1y developed and free-swimming heads without any trunk-—as representing ancestral forms (of. Graham Kerr, 1911).
In considering whether a particular stage of development is to be taken as probably repeating an ancestral stage of the adult special attention should be directed towards its mode of life, with the object of estimating the degree to which it diverges from the probable mode of life of the ancestral stage. If its mode of life is strikingly aberrant, ag. parasitic where the normal habit of the group is free-living, or pelagic where the normal habit is not pelagic, then we must always keep in mind the possibility or probability that its most conspicuous features are mere modern adaptations and are therefore worthless as evidence of ancestral conditions.
Again it should be considered whether in the main features of its organization it agrees with animals which are admittedly allied to it.
Larvae occur in the following Vertel;rates———Amp7z/ioasus, Petromg/zon, Crossopterygians, Ganoids, many Tcleosts, Lung-fishes and the majority of Amphibians. Applying such criteria as are indicated above we should rule out as probably devoid of phylogenetic significance the larva of A7nph'i0;L'u.s’ on account of its quite aberrant “pleuronectid” asymmetry (see Vol. I. Chap. XVII.). We should again rule out the Teleostean larvae on account of their extreme diversity. In Urodele Amphibians and Dipneumonic Lung-fishes on the other hand we see larvae which appear to be distinctly of a common type. And in Crossopterygian and Actinopterygian Ganoids we again find larvae which differ from these in detail rather than in fundamental characteristics. Consequently we should incline towards the view that the type of larva in question does not depart very widely from the common ancestral type out of which existing Vertebrates have evolved.
Again in considering whether a particular feature of structure is to be regarded as ancestral or as a modern adaptation the following questions should be asked: (1) Is the feature peculiar to one group of Vertebrates or does it occur in several groups, and (2) if it occurs in several groups do the various animals possessing the peculiarity in question undergo their larval stages in similar sets of environmental conditions?
If the particular feature occurs in several groups derived from a common ancestral form this obviously increases the probability of the feature itself being ancestral. If however the several groups show similar sets of environmental conditions during their larval stages this introduces the element of doubt whether the similar features may not after all be merely adaptations to these similar sets of conditions.
Again it is important to make out whether the particular similarity has to do merely with parts of structure in direct functional relationship to external conditions. If there be deep-seated correspondences in structure with no such direct functional relationship to external factors then this gives greatly increased probability to these correspondences being truly ancestral in their nature.
The morphologist in trying to decipher the record of evolutionary history from the data of comparative anatomy or embryology is constantly impressed by the potency of nature’s economy of living substance. An organ no longer required may be eliminated within a very short period of evolutionary time. Thus in some species of Mackerel (Scombe7') so important an organ as the air-bladder has been eliminated: in various Frogs and Toads the external gills have been eliminated from development. Thus negative embryological evidence is of peculiarly little weight in relation to phylogenetic problems.
5. The Protostoma Hypothesis
This is a working hypothesis which links together and in a sense explains a number of features in the early development of Vertebrates which are otherwise extremely puzzling. The more important of these features may be summarized as follows:
In Amplmloruus as has already been shown the dorsal side is at first occupied by the widely open gastrular mouth. Later this becomes roofed in by a backgrowth of the gastrular rim anteriorly. A similar process of backgrowth appears to take place in the gastrulation of lower Vertebrates in general. The roof of the gastrular cavity formed by this process gives rise later not merely to the dorsal wall of the alimentary canal but also to notochord and central nervous system.
I. N ow occasionally there are appearances which suggest that this archenteric roof consists really of two lateral halves which have become fused together along the sagittal plane. Thus in Protopterus the down-growing dorsal lip of the blastopore is frequently indented by a median incision. Again in Urodeles the medullary plate is frequently traversed by a fine superficial groove wh.ich passes forwards along the median line.
II. Then there occur curious cases of abnormality in which the dorsal region of the body is actually divided into two halves by a longitudinal split in the mesial plane. Thus Oscar Hertwig found (1892) that by fertilizing frog’s eggs which had become over-ripe, either by retention within the oviduct or by being kept for from one to four days in a moist chamber, he obtained a certain number of abnormal embryos of the type shown in Fig. 220, A, where a large expanse of yo1k—cells is visible in dorsal View instead of being completely covered in as would be the case normally. In transverse section (Fig. 220, B) such an embryo was found to have two half neural rudiments and two notochords of half the usual size, widely separated by the mass of yolk or endoderm.
FIG. 220. -—Abnorxnal embryos illustrating the Protostoma theory.
A, abnormal Frog embryo seen from the dorsal side; B, transverse section through hinder third of ditto (after 0. Hertwig, 1892); C, abnorma.1'I‘ront embryo (S. famio) in dorsal view (after Kopsch, 1899); D, ahnorxnal embryo of Pike (Esox lucius) (after Lereboullet, 1863). m. f, medullary fold ; m.s, mesoderm segment; mes, mesoderm; N, notochord; 0, opening leading down into enteron; ot, otocyst; 3/,'_1nass of yolk-cells.
Similar abnormalities have been observed in Teleostean fishes. E.g. Fig. 220, D shows a Pike-embryo which is normal towards its anterior and posterior ends but interrupted for some distance in the mid-dorsal line by a Wide cleft in which the yolk is visible. Again in Fig. 220, C a similar cleft" is seen to traverse the whole length of a Trout-embryo from the hind-brain region tailwards.
An important feature of such abnormal embryos of fish and amphibians is that they frequently proceed with their development, the lips of the fissure closing up and the two sets of half-organs being brought together in the mesial plane, undergoing complete fusion and the individual becoming in fact entirely normal. The importance of this return to the normal on the part of such split embryos is that it indicates that the departure from the normal during the split condition is far less fundamental than would appear at first sight.
Here then we have to do with two very remarkable phenomena. Firstly there is the abnormality itself—-—the fact that the dorsal region of the body is for a time in the form of two distinct halves. Abnormalities of such a definite type as this usually have a definite evolutionary or other meaning and it is necessary to search for such a meaning in this particular case. Secondly, there is the fact that an embryo almost completely bisected in this way is frequently able to right itself and become perfectly normal. This again suggests the question whether this power of righting itself has not some special evolutionary meaning.
III. In the higher meroblastic Vertebrates we have seen that there exists along the middle line of what corresponds with the archcnteric roof of Avnplzxioccus (’i.8. the region which becomes converted into the dorsal part of the body, including notochord and medullary plate) the structure known as the primitive streak. We have also seen that in the lowest Vertebrates possessing it, this primitive streak represents the line of fusion of the gastrular lips, and that we are therefore justified in attaching the same significance to the primitive streak in those higher forms in which the actual process of fusion can no longer be observed. That this interpretation is correct is indicated by the occasional occurrence of openings in the line of the primitive streak communicating ventrally with the enteron and dorsally with the outer surface of the medullary plate, or its derivative the floor of the neural tube (pp. 51, 53). Such neurenteric communications are readily explicable by the view that they represent simply parts of the line of fusion of the gastrular lips where the actual fusion has not been completed. Here again we have a phenomenon which demands explanation-——the occurrence of what seems to be the vestige of a slit-like gastrular mouth along the middorsal line.
IV. We have another remarkable body of facts associated with the fate of the blastopore or remnant of the gastrular mouth in various groups of the animal kingdom; Thus within the limits of the groups Annelida or Mollusca the blastopore in some forms becomes the mouth, in others the anus. N 0 one would doubt for a moment that the mouth opening is homologous throughout these groups yet in one member of the group it can be traced back to the blastopore while in another member it is the anus which can be so traced. In other forms the gastrular mouth simply vanishes away during development and in some of these cases it assumes a curious elongated slit-like form along the mid-neural line before it disappears.
It is the merit of the Protostoma theory that it—and it alone—— affords an explanation of these four very different but equally puzzling bodies of facts. It falls therefore to be accepted by the Vertebrate embryologist as one of his working hypotheses.
The Protostoma theory is simply a special development of the theory of the evolution of the coelomate Metazoa which is generally accepted by morphologists, namely that the animals in question have passed, during the remoter parts of their evolutionary history, through a Protozoan and later a Coelenterate stage. The peculiarity of the Protostoma theoryis that it includes
"FIG. 221.-—~View of neural rudiment in embryo of within the coelenterate period
{;6’i'lZ}1(ltllu$’ Sedgwiclp, ; '8.Il(l. a Stage correspondillg in Cpl, 081/F871;. I1 1e Case 0 epz. osrren le - - embryo is shown as it appears when straight- maln Structural matures Wlth
ened out. the Actinians of the present time, characterized by the presence of an elongated slit-like mouth, dilated somewhat at each end and surrounded by a specially concentrated portion of the ecto dermal nerve plexus. The portion of the surface on which the slit-like mouth was situated was thus the neural surface.
Sedgwick (1884) was led to the idea by his studies on the development of Peripatus. He found in the species investigated by him a stage (see Fig. 221, A) in which the gastrula-mouth formed a long slit traversing the neural surface and surrounded by the ectodermal neural rudiment. As development went on the gastrular mouth or protostoma became obliterated, except in its dilated terminal portions, by fusion of its lips. The terminal parts remained open as mouth and anus respectively. The portions of nerve rudiment between the two openings became the ventral nerve cords while the portions in front of the mouth and behind the anus gave rise respectively to the IX _ PROTOSTOMA THEORY 497
supra-oesophageal ganglia and the suprareetal commissure. According to Sedgwick this stage in the development of Pervlpatus repeats the features of an Actinozoon-like Coelenterate ancestor, not merely of Pemlpatus, and therefore of Arthropods in general, but of such other groups as Annelids, Molluscs and Vertebrates.
It will be noted that on this protostoma hypothesis an important physiological distinction has at an early period of evolution marked off the Vertebrates from the other groups mentioned. This distinction came about with the acquisition of different habits of movement. In the stem which gave rise to Annelids, Arthropods, Molluscs, movement took place with the neural surface next the substratum (as in those modern Medusae which are able to creep on a solid surface-—e.g. Cladonerna), while in the Vertebrate stem on the other hand the neural surface was directed away from the solid substratum (as in the modern Actinian when it creeps). This difference in the position of the body in relation to the substratum would naturally lead in time to the different types of dorsiventrality so apparent in the fundamental organization of the two diverging stems. It is frequently stated by critics of the protostoma hypothesis that it involves a reversal of dorsal and ventral sides during the evolution of Vertebrates from their invertebrate ancestors but it will be gathered from what has been said that this criticism rests on a misunderstanding.
It will be readily seen that the protostoma hypothesis successfully explains the four categories of puzzling facts already enumerated. The paired appearance of the gastrular roof would be a reminiscence of the fact that originally it was actually paired: the split along the back of the abnormal embryos would mean the temporary re-appearance of the ancestral split or mouth: the primitive streak would be the scar along which the lips of this ancestral mouth or protostoma underwent fusion: and the converting of blastopore now into mouth now into anus would he an imperfect reminiscence of the fact that in phylogeny it gave rise to both.
On this hypothesis the various signs of a split along the neural surface of the vertebrate embryo, whether in the form of a dorsal furrow or a primitive streak or an actual opening, areinterpretable as reminiscences of the protostoma slit which traversed the neural surface of the Actinozoon-like ancestor.‘ It is of interest to notice that in two Vertebrates at least there exist what seem to be obvious traces of neural rudiment extending round behind the anal part of the protostoma precisely as in Peripatus. In Fig. 221, B, is shown an embryo of Lepidosiren spread out in one plane, with the neural rudiment in the form of a ridge which is continuous behind the blastopore or anus. If it be reflected that this opening may be regarded as being continued forwards by a potential slit, represented eg. by the primitive streak of other forms, it will be realized how close is the resemblance to the conditions in Peripwtus. The pre-anal portions of the neural rudiment in Lepidosiren come together in the mesial plane to form the spinal cord, while the postanal portion flattens out and disappears so that theanal opening comes to lie entirely behind the posterior limit of the central nervous system. It is clear that if the development of the anal opening were delayed until the neural folds had already come together it would make its appearance completely behind the central nervous rudiment and with no obvious connexion with it. This is very possibly the case in Vertebrates other than those mentioned.
- 1 That the primitive streak and primitive groove are closely related to the gastrula mouth was perceived by Rauber (1877) but a clear evolutionar explanation of this relationship was first given by the protostoma theory of Sedgu 1c (1884) and Hertwig (1892)
Although the anal opening of Vertebrates is thus brought into the relations with the nervous system that we should expect on the protostoma hypothesis there is no such definite evidence in the case of the mouth. It is true that in some cases the dorsal furrow has been traced to the neighbourhood of the mouth and that the mouth opening has in some cases at first the form of a sagittally placed slit, but in no ease, up to the present, has the neural rudiment been traced round in front of the mouth. This difficulty however is greatly lessened when We correlate the facts just mentioned in regard to the anal opening in Lepidcisiren with the relatively late appearance of the mouth opening of Vertebrates as discussed on p. 193. It may well be that the non-inclusion of the mouth opening within the obvious neural rudiment is due simply to the pre-oral parts of the medullary folds having already flattened out and disappeared before the oral opening makes its appearance. If this is the case it carries with it the interesting consequence that the supra-oesophageal or pre-oral ganglia of I’emIpatus have disappeared in the Vertebrate and it is therefore waste of energy to discuss what parts of the brain of a Vertebrate are homologous with the supraoesophageal ganglia of Invertebrates.
This Protostoma idea, dealing as it does with extremely remote phases of the Vertebrate phylogeny, must not be looked on as a definitely proved theory, nor can it be expected ever to reach that dignity, but it is a fascinating working hypothesis which serves, and which alone serves, to link together and in a sense explain a considerable body of otherwise mysterious and apparently inexplicable facts of Vertebrate embryology.‘
6. The Vertebrate Head
The two phyla of the animal kingdom which have reached the highest stage of evolutionary deve1opment—the Arthropoda and the Vertebrata——are alike characterized by the possession of a well-developed head. In the evolution of a head we may take it that the principal factors involved are probably the following:
(1) The habit of active movement in a direction corresponding with the prolongation of the axis of the body,
(2) The concentration of organs of special sense towards the end of the body which is in front during movement,
(3) The concentration of nerve centres to form a brain in proximity to these organs of sense.
- 1 In considering the difficulties in the way of the theory afforded by cases where the gastrula becomes roofed in by a process of simple backgrowth without any trace of protostoma. (e.g. Amphiomus), it is well to bear in mind the parallel case of the amnion-——of which a large portion may be formed by simple backgrowtb, although the sero-amniotic isthmus and the ingrowth of mesoderm from the two sides seem to point clearly to a former formation by the meeting of two lateral folds.
In the case of the Vertebrate the brain has reached a comparatively large size and in correlation with this the protecting skeleton has become highly developed and has lost the flexibility which is characteristic of it in the trunk. Further in the Vertebrate the walls of the buccal cavity and pharynx have become highly specialized, particularly in the matter of their skeleton, in relation to the functions of ingestion and mastication of the food on the one hand, and of respiration on the other.
Each of these various factors involves structural change, not affecting merely one organ but causing modification of the whole complex arrangements of the head-region. Thus associated with the loss of flexibility we find (1) loss of segmentation of the skeleton, (2) disappearance or great modification of the myotomes, (3) corresponding changes in the nerves supplying these myotomes and (4) disappearance of the coelomic cavities.
The full appreciation of the importance of this feature of the Vertebrata makes it, in the present writer's opinion, impossible to doubt that the possession of a definite head is a feature that has come down from the unknown ancestral form from which the Vertebrate stock has evolved. If this be correct it follows that the relatively feeble differentiation of the head end of the body seen in Amplmioams is to be regarded as a secondary condition, correlated with the peculiar mode of life of this animal, and devoid of phylogenetic significance.
It has already been pointed out that organs of great complexity in the adult tend to be laid down at an early stage of individual development, time being thus obtained for the development of their complex detail. It is perhaps in direct relation to this principle that the highly complex head - region of the Vertebrate, which comes to assume control over most of the activities of the individual, develops particularly early in ontogeny ——the Various developmental processes making themselves as a rule first apparent in the head region and spreading thence tailwards along the trunk. This fact is of practical importance to the embryologist for in the case of segmentally repeated organs it enables him to find a series of developmental stages within the body of a single embryo.
Though this precocious cephalization is a marked feature of Vertebrate ontogeny it never goes within this phylum to the length it does amongst certain Invertebrates where the larval stage (N auplius, Trochosphere) is practically a precociously developed and free-living head which has not yet developed a trunk.
As will have been gathered, one of the most conspicuous features of the head-region is the loss of segmentation in organs in which it was once present.
Metamerie segmentation, which first makes its appearance in typical form in the Annelida, is probably to be associated primarily with the coelome and its lining the mesoderm. The coelome is distended with coelomic fluid and the turgidity so caused gives firmness to the body. The physiological advantage of the coelomic cavity being subdivided into successive compartments is obvious. The segmentation of other organs is to be looked on as secondary to that of the mesoderm, and more especially to that of the muscles. Thus the segmented character of the nervous system of an Annelid or Arthropod is due to the ganglion-cells tending to become concentrated at the level of the masses of muscle which work the parapodia or limbs. So also the segmentation of the skeleton which permits flexure of the body is correlated directly with the segmentation of the musculature which causes that fiexure.
So, conversely, with the disappearance of segmentation in the head of the Vertebrate. Correlated with the loss of flexibility in the brain region the myotomes which produce the flexure have disappeared, and correlated with this in turn the ensheathing skeleton has lost its segmentation and the segmentally arranged motor nerves have also gone. The process has taken place from before backwards. It has been carried to the greatest extent in front, to the least at the hinder limit of the head.
It is definitely established that the head of the Vertebrate has at least in part come into being by the modification of what was once the anterior portion of the trunk. With the gradual evolution and increase in size of the brain—-so characteristic of the phylum Vertebrata—this organ has gradually encroached upon the spinal cord, and its protective skeleton the chondrocranium has pare} passu encroached upon the vertebral column. This is clearly indicated by the fact that included within the limits of the skull are nerves which are serially homologous with those of the trunk. Putting on one side the probability-as many would regard it— that cranial nerves III, IV, V, VI, VII, IX and X are really homologous with the spinal nerves, we find behind the Vagus a series of spino-occipital nerves (Fiirbringer, 1897), which although included within the limits of the skull are yet undoubtedly members of the same series as the spinal nerves. The number of those is very different in the different subdivisions of the Vertebrata as may be gathered from aninspection of Fig. 222. In all probability they will be found also to show considerable variation in different individuals of the same species.
During the evolution of the head there is some reason to believe adult stage of modern Cyclostomes: it is also seen in the young Lepidosiren of stage 34 (see Fig. 154, B, p. 309).
The next phase is seen in the adults of such relatively primitive groups as the Elasmobranchs, the dipneumonic Lungfishes and the Amphibians, in which an occipital region has been added on to the palaeocranium.
FIG. 222.-—-Diagram illustrating the relations of the binder limit of the cranium in an Elasmobranch (A), Lepidosiren (B), Polypterus (C), Acipenser (D), an Amphibian (E), and a Reptile (F), as seen from the left side.
The cranial floor is indicated by the broad horizontal black band : it is demarcated from the vertebral column by the vertical band which represents the occipital limit of the cranium. Dorsal and ventral nerve roots are shown as black dots, when transitory as rings, when occurring only occasionally as dotted rings. The myotomes are indicated by rectangular outlines. The Spinooccipital myotomes are lettered according to Furbringer's system—the anterior batch (occipital) with the concluding letters_of the alphabet, the posterior batch (occipito-spinal) with the commencing letters (A, B, C’). Trunk myotomes not yet incorporated in the head are designated by numbers.
that its extension backwards has taken place by successive steps. In the most ancient recognizable stage the cranium (Palaeocranium ——Fiirbringer) extended no farther back than the vagus nerve. This phase is represented--either persistent or revertive——in the
Finally the hinder limit in the other Vertebrates has been shifted still farther back—one segment (Poly/ptems), three segments (Amniota) or as many as five segments (Acipenser).
As far forwards as the hinder limit of the palaeocranium there is, as already indicated (p. 317), clear evidence that the cranial wall represents a series of neural arches which have undergone fusion. As indicated on the same page it is difficult to avoid extending this homology to the mesotic portion of cranial wall lying still farther forwards. As regards the prechordal portion of the cranium there is no definite evidence, but if we regard the trabeculae as primitively in continuity with the parachordals we have to grant the possibility of even this part of the cranium being in series with the portions farther back and therefore also originally vertebral in constitution.
In conclusion it must be remembered that the series of myotomes is also continued into the head-region, and the occurrence of typical myotomes as far forwards as the premandibular or oculomotor segment (p. 210) may be taken as strong evidence that the segmentation of the mesoderm originally extended throughout the head-region including its pre-chordal portion.
7. Embryology and the Evolution of the Vertebrate
The special charm and the chief importance of the study of embryology reside in the fact that it is one of the main branches of evolutionary science. The greater part of what is ordinarily called evolutionary research deals with the possible methods and causes of evolutionary change. The data of Embryology on the other hand form a branch of synthetic evolutionary science which deals not with possible causes or methods but with the actual facts of evolutionary change, striving to map out the course along which this has proceeded. ln compiling the record of evolutionary progress we are dependent upon Comparative Anatomy and Palaeontology as well as Embryology, and in formulating conclusions care has to be taken that whenever possible they are based on the data of all three sciences. In cases where these data are not in agreement care must be taken to bear in mind the main disturbing factors which are liable to invalidate the conclusions in each case. In reasoning from Embryology and Comparative Anatomy the possibility that particular features are modern adaptations to existence say within a uterus or egg-shell or under any other set of conditions different from those of the ancestor has to be borne in mind. In the case of Palaeontology and Comparative Anatomy there exists the same danger of error as besets the protozoologist when he endeavours to construct a continuous life-history out of a number of isolated observations on the dead animal—the error of arranging observations in a series which is not natural, or on the other hand, if the seriation be done correctly, of reversing its direction. In Palaeontology errors of this type are peculiarly apt to arise on account of the extraordinary imperfection of our knowledge. If a- series of organisms a, b, c, d, become known from a series of geological deposits A, B, 0, D, this affords convincing evidence in most cases that the particular organisms lived at the time the particular deposits were laid down: the conclusion may also be fairly justifiable that not only did they exist but that they were abundant at the period in question. The conclusion however which is so apt to be drawn that w, b, c, d, actually made their first appearance in the same order as the deposits A, B, 0, 1), is quite unreliable. They may have existed in smaller numbers for immense periods of time before the periods corresponding to A, B, 0’, 1), when they were really abundant, and the order of their first appearance may have been d, c, b, a, or any other. Such a geological series is in fact in itself of little value as an index to the order of evolution. In Embryology on the other hand where the evolutionary stages occur as part of a continuous process, each dependent upon its predecessor, we appear to be safe in assuming that the record, however incomplete, is at least arranged in proper sequence.
Another principle to be borne in mind, when the attempt is being made to work out the evolutionary history of a particular group‘ is that conclusions must be based upon broad knowledge of structure as a whole. No implicit reliance must be placed upon evidence relating to one system of organs unless it is corroborated by the evidence of other organs. Failing this precaution the investigator is liable to the pitfall afforded by convergent evolution of organs of similar function. Here again the palaeontologist finds himself much hampered as compared with the embryologist, for as a rule all evidence except that of the skeletal system has passed completely beyond his ken.
EVOLUTIONARY ORIGIN or THE VERTEBRATA.
In the preceding portions of this book it has been shown that Embryology provides us with a record in at least its main outlines——ot' the evolutionary changes which the various organ-systems have undergone within the group Vertebrata. For, amongst others, the reasons stated at the foot of p. 491 the record is less clear regarding the evolutionary history of the complete individual. Even however if we had this record complete for the various types—Fish, Amphibian, Reptile, Bird—we should find ourselves still confronted with the interesting problem of the first origin of the primitive Vertebrate type :—from whence came these lowly original Vertebrates out of which the various existing types of Vertebrate have been evolved?
This problem of the ancestry of the Vertebrata is naturally a fascinating one and it has attracted much attention and been the theme of voluminous writing. Enthusiasts have at different times endeavoured to demonstrate that the Vertebrates are descended from this phylum or from that. It is perhaps best not to take such attempts very seriously. They have served a useful purpose in arousing interest and stimulating research but they have little claim to a place in the permanent literature of Zoology. «
We are naturally unable to get any evidence bearing upon the problem from Palaeontology. The most ancient Vertebrates of which fossil remains are known had probably already evolved to a far greater distance from the original type of Vertebrate than that which separates them from the existing Vertebrates of to-day. And the probability is that the earliest Vertebrates went on existing and evolving through long ages before they developed those complex skeletal structures which are alone adapted for preservation as fossils in the geological record. Comparative Anatomy fails us too——-for up to the present no existing type of animal has been discovered which can justifiably be interpreted as an unmodified survivor of the original Vertebrates.
It is Embryology alone which yields us examples of Vertebrates in the earliest stages of evolution but the data afforded by that science do not carry us beyond the formulation of a few very broad and general conclusions regarding the prevertebrate phases in the evolutionary history of the phylum.
(1) The fact that Vertebrates, like other Metazoa, commence their existence as a unicellular zygote appears to justify us in postulating a unicellular tie. a Protozoan ancestral stage.
(2) The fact that there occurs in the admittedly more primitive Vertebrates a gastrula stage appears to justify us in postulating a diploblastic or Uoelenterate ancestral stage.
(3) The facts which are united together in the Protostoma hypothesis suggest that the coelenterate ancestor evolved along lines somewhat similar to those of the modern Sea—anemones with their elongated slit-like protostoina dilated at each end and surrounded by a concentration of the ectodermal nerve-plexus.
(4) The facts that the coelome was probably originally segmented (as indicated by Amphtowus), that the excretory organs are in the form of nephridial tubes, that the vascular system consists fundamentally of longitudinal vessels on opposite sides of the alimentary canal connected together by vascular arches, the blood passing tailwards in the vessel on the neural side of the alimentary canal—— suggest that there intervened between the coelenterate phase and the vertebrate phase a stage which possessed many features in common with those animals which are grouped together to-day in the phylum Annelida. We may suppose that this annelid-like creature became evolved from an Anemone in which the body had become drawn out, as in the genus Herrpolitha or one of the brain corals, and which had become actively motile. In the two diverging stems which gave rise to Annelids and to Vertebrates respectively we may take it that a difference existed in the normal position of the body—-—the former progressing with their neural, the latter with their abneural, surface underneath. It is conceivable that this difference may have been associated with the difference between a creeping mode of life in which the chief sensory impressions were related to the solid substratum and a swimming mode of life in which they rather came from above. Ix GERM LAYER THEORY 505
Addendum to Chapter IX
More than once in the course of this volume reference has been made to the “ Theory of Germinal Layers” or the “Germ Layer Theory.” This theory, which has played a great part in the development of embryological science in the past and still dominates to a great extent embryological research, had its foundations in observations made by these pioneers of embryological science — Wolff, Pander, von Baer and Remak. Wolff (1768) observed that the alimentary canal in the Bird embryo is developed out of a thin membrane or leaf (“ Blatt”) and inferred that the other organs go through a similar stage. Pander (1817) gave the name “ blastoderm ” to the first membrane-like stage of the embryo as a whole, saw how this became differentiated into the three layers-——outer, middle and inner——and traced out the development from _these of the main organ-systems. Von Baer (1828) carried on and elaborated l’ander’s work, recognized that the middle layer was double, and that it was secondary to the two primary layers: the outer and the inner. He also extended his observations to forms other than the Fowl and laid the foundations of Comparative Embryology. Remak (1855) finally worked out the germ-layers in terms of the Cell-theory, traced the origin of the coelome to a split in the middle layer, and worked out more precisely the relations of the layers to the definitive organ-systems.
One of the most important steps in the development of the Germ Layer Theory was made by Huxley (1859) who as a result of his researches upon the Medusae recognized the two primary cell-layers in these animals (named by Allman “ eetoderm ” and “ endoderm ”) and suggested the comparison of them with the two primary layers of the Vertebrate embryo.
Embryology, like Morphology in general, first became a real living science as a result of Darwin’s demonstration of the fact of evolution. In the Omlgivt Qf Species (1859) the principle of recapitulation is already admitted. “Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals.” The idea was further elaborated by Fritz Muller (1864).
Kowalevsky (1871, etc.) and other embryologists had demonstrated the wide-spread occurrence among the Invertebrates of an early stage of development more or less cup-shaped in form and consisting only of the two primary cell-layers, and the important advance was made synchronously by Lankester and Haeckel of perceiving in this two-layered stage a repetition of a common ancestral form.
Lankester (1873) recognized amongst the Metazoa two distinct grades of complexity of structure so far as their cell-layers were concerned—the diploblastic grade (represented by the Coelenterate) consisting of the two primary layers, and the triploblastic grade with an interposed middle layer. Further he recognized that each Metazoon—whatever its definitive condition—passes in the course of development through a diploblastic stage which he termed the planula. Such a planula. stage he regarded as a repetition of a common ancestral stage of evolution.
Haeckel (1872) about the same time as Lankester also developed the idea that the diploblastic stage of ontogeny was to be interpreted as the repetition of an ancestral form: Haeckel called this ancestral form Gastraea. The main difference between Haeckel’s view and Lankester’s was that the former regarded the endoderm as having arisen by a process of invagination——-as it actually does arise in ontogeny in the great majority of cases—-—while Lankester regarded it as having arisen by a process of delamination from the outer la er.
y As regards the middle germ-layer ideas remained somewhat vague until Agassiz (1864) showed that in the Starfish the mesoderm arose in the form of an outgrowth of the archenteric wall. The same was found to be the case in various other Invertebrates, and in 1877 Kowalevsky showed how in Amp}:/iowus the mesoderm was during an early stage in the form of archenteric pockets. In the same year Lapkester developed the generalization that the coelome is to be regarded as uniformly enterocoelic in origin and comparable with the diverticula of the archenteric lining seen in Coelenterata.
The separation of such mesodernial cells as are in their early stages free and amoeboid under the common name mesenchyme was first made by O. and R. Hertwig (1882).
The later developments of the theory of the mesoderm involved in the Protostoma theory have already been alluded to earlier in this volume and the same applies to what the author regards as the chief qualification of the germ-layer theory indicated by modern work, namely that the boundary between two layers where they are continued into one another must be regarded not as a sharply marked line but as a more or less broad debatable zone.
Literature
Agassiz. Contributions to the Natural History of the United States of America, v. Boston, 1864. [Vol. v printed as vol. v, pt. 1, of Mem. Mus. Comp. Zoology H8l'Va.1‘d.] _
Baer. Uber die Entwickclungsgeschichte der Thiere. Beobachtung und Reflexion, i. Konigsbcrg, 1828.
Bell. Arch. Entwick. Mechanik, xxiii, 1907.
Piirbringer. Gegenbaurs Festschrift. Leipzig, 1897.
Haeckel. Die Kalkschwamme. Berlin, 1872.
Hertwig, 0. Arch. mikr. Anat., xxxix, 1892.
Hertwig, 0. and R. J enaische Zeitschrift, xv, 1882.
Jérgensen. R. Hertwigs Festschrift. Jena, 1910.
Kerr, Graham. Proc. Roy. Phys. Soc. Edin., xviii, 1912.
Kopsch. Internat. Monatsschr. Anat. u. Phys., xvi, 1899.
Kowalevsky. Mém. Acad. Sci. St-Pétersbourg, (7), xvi, 1871.
Kowalevsky. Arch. mikr. Anat., xiii, 1877.
Lankester. Ann. Mag. Nat. Hist., (4), xi, 1873.
Lankoster. Quart. Journ. Micr. Sci., xvii, 1877.
Lereboullet. Ann. Sci. Nat., 4, Zool., xx, 1863. IX GERM LAYER THEORY 507
Mfiller. Fiir Darwin. Leipzig, 1864. ‘ Pander. Beitrzige zur Entwickelungsgeschichte des Hiihnchens im Eye. Wiirzburg, 1817.
Parker. American Natltralist, xliii, 1909.
Rabl. Morph. Ja.hrb., xv, 1889.
Rauber. Priniitivstreifcn und Neurula. der Wi1'bc1thie1'e. Leipzig, 1877.
Remak. Uxltersuchuugen ulwr die Entwicke-lung der \Virbe1thiere. Berlin, 1855. Sedgwick. Q,11a.rt. Journ. Mic-r. Sui., xxiv, 1884.
Sedgwick. Quzwt. Jmxrn. Micr. Sci., xxxvii and xxxviii, 1895 and 1896.
Wolff. Nov. Conmwnt. Acad. Sci. St-I’éte1'Sl>O1U'g. Xii, 1768.
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- Currently only Draft Version of Text -
Textbook Chapters: 1 Formation of the Germ Layers | 2 Skin and Derivatives | 3 Alimentary Canal | 4 Coelomic Organs | 5 Skeleton | 6 Vascular | 7 Internal Body Features | 8 Adaptation to Environmental Conditions | 9 General Considerations | 10 Common Fowl | 11 Lower Vertebrates | Appendix
Reference
Kerr JG. Text-Book of Embryology II (1919) MacMillan and Co., London.
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