layer) will give origin to the animal (neuromuscular) part of the body, the lower pair to the plastic or vegetative organs. The uppermost layer will form the external covering of the embryo, and also the amniotic folds; from it there differentiates out at a very early stage the rudiment of the central nervous system, forming a more or less independent layer. Below the outermost layer lies the layer from which are formed the muscular and skeletal systems, and beneath this "muscle-layer" comes the "vessel-layer," which gives origin to the main blood-vessels. The innermost layer of the four will form the mucous membrane of the alimentary ca.n.a.l and its dependencies; at the present stage, however, it is, like the other layers, a flat plate.

From all these layers tubes are developed by the simple bending round of their edges. The outermost layer becomes the investing skin-tube of the embryo; the layer for the nervous system forms the tubular rudiment of the brain and spinal cord; the mucous layer curls round to form the alimentary tube; the muscle layer grows upwards and downwards to form the fleshy and osseous tube of the body wall; even the vessel layer forms a tube investing the alimentary ca.n.a.l, but a part of it goes to form the medial "Gekrose," or mesenterial complex, which departs considerably from the tubular form.

When these tubes or "fundamental organs" are formed the process of primary differentiation is complete. The fundamental organs, however, have at no time actually the form of tubes; they exist as tubes only ideally, for morphological and histological differentiation go on concurrently with the process of primary differentiation.

Through morphological differentiation the various parts of the fundamental organs become specialised, through unequal growth, first into the primitive organs and then into the functional organs of the body. "Single sections of the tubes originally formed from the layers develop individual forms, which later acquire special functions: these functions are in the most general way subordinate elements of the function of the whole tube, but yet differ from the functions of other sections. Thus the nerve-tube differentiates into sense-organs, brain and spinal cord, the alimentary tube into mouth cavity, oesophagus, stomach, intestine, respiratory apparatus, liver, bladder, etc. This specialisation in development is bound up with increased or diminished growth" (p. 155). Rapid growth concentrated at one point brings about an ev.a.g.i.n.ation; in this manner are formed the sense-organs from the nerve-tube, the liver and lungs from the alimentary tube. Or increased growth over a section of a tube causes it to swell out; in this wise the brain develops from the nerve-tube, the stomach from the alimentary tube. The segmentation which soon becomes so marked, particularly in the muscle layer, is also due to a process of morphological differentiation.

At the same time that the organs of the body are being thus roughly blocked out and moulded from the germ-layers the third process of differentiation is actively going on. "In addition to the differentiation of the layers, there follows later another differentiation in the substance of the layers, whereby cartilage, muscle and nerve separate out, a part also of the ma.s.s becoming fluid and entering the bloodstream" (p. 154). Through histological differentiation the texture of the layers and incipient organs becomes individualised. In its earliest appearance the germ consists of an almost h.o.m.ogeneous ma.s.s, containing clear or dark globules suspended in its substance (ii., p. 92). This h.o.m.ogeneity gives place to heterogeneity; the structureless ma.s.s becomes fibrous to form muscles, hardens to form cartilage or bone, becomes liquid to form the blood, differentiates in a hundred other ways--into absorbing and secreting tissues, into nerves and ganglia, and so forth. It will be noticed that the concept of histological differentiation is independent of the cell-theory; it signifies that textural differentiation which leads to the formation of tissues in Bichat's sense. The tissues and the germ-layers stand in fairly close relation with one another, for while certain tissues are formed chiefly but not exclusively in one layer, others are formed only in one layer and never elsewhere. For example, peripheral nerves are for the most part formed in the muscle layer, though the bulk of the nervous tissue is formed in the walls of the nerve tube; similarly blood and blood-vessels may arise from almost any layer, though their chief seat of origin is the vessel-layer; on the other hand, bone is formed only in the muscle-layer (i., p. 155, ii., pp. 92-3).

This relation of tissue to germ-layer was more fully discussed and brought into greater prominence by Remak, from the standpoint of the cell-theory, and it will occupy us in a later chapter (Chap. XII.).

The fourth Scholion elaborates the a.n.a.lysis of developmental processes still further, and discusses in particular the scheme of development which is shown by the Vertebrata. The characteristic structure of the vertebrate body is brought about by a "double symmetrical" rolling together of the germ-layers, whereby two main tubes are formed, one above and one below the axis of the body, which is the chorda. The dorsal tube is formed by the two animal layers, the ventral tube by all the layers combined (see Fig. 7).

The process is indicated with sufficient clearness in the diagram. It will be seen that the real foundation and framework of the arrangement is the muscle-layer, with its two tubes, one surrounding the central nervous system and forming the "dorsal plates," the other surrounding the body cavity and forming the "ventral plates." In the dorsal plates, which early show metameric segmentation, the investing skeleton of the neural axis develops; in the ventral plates are formed the ribs, the ventral arches of the vertebrae, the hyoid, the lower jaw and other skeletal structures.

The alimentary or "mucous" tube and the part of the vessel layer which invests it become so closely bound up with one another as to form a single primitive organ--the alimentary ca.n.a.l. The muscles of the alimentary ca.n.a.l are accordingly in all probability developed in the investing part of the vessel layer. From the "Gekrose," or remaining part of the vessel layer develop the Wolffian bodies (_Urnieren_, p.r.o.nephros), the kidneys, the s.e.x glands, and the series of "blood-glands"--suprarenals, thyroid, thymus and spleen. Baer did not attach any special morphological significance to the peritoneal lining of the body cavity, as is done in more modern forms of the germ-layer theory. The gill-slits were largely formed by outgrowths from the alimentary ca.n.a.l.

_a._ Chorda.

_b._ Dorsal plates.

_c._ Ventral plates.

_d._ Spinal cord.

_e._ Vessel-layer.

_f._ Alimentary tube.

_g._ p.r.o.nephros.

_h._ Skin.

_i._ Amnion.

_k._ Serous membrane.

_l._ Yolk-sac.

In his germ-layer theory von Baer was influenced a good deal by Pander, to whom the actual discovery of the process of layer-formation is due. Pander, however, had distinguished only three germ-layers, an upper "serous" layer, a lower "mucous" layer and a middle "vessel-layer." He it was who introduced the terms "Keimhaut"

(blastoderm) and "Keimblatt" (germ-layer).

[Ill.u.s.tration: FIG. 7.--Ideal Transverse Section of a Vertebrate Embryo.

(After von Baer.)]

The honour of being the founder of the germ-layer theory is sometimes attributed to C. F. Wolff, notably by Kolliker and O. Hertwig. Wolff, it is true, in his memoir _De formatione intestinorum_ (1768-9) showed that the alimentary ca.n.a.l was first formed as a flat plate which folded round to form a tube, and in a somewhat vaguely worded pa.s.sage he hinted that a similar mode of origin might be found to hold good for the other organ-systems. But it seems clear that Wolff had no definite conception of the process of layer-formation as the first and necessary step in all differentiation. This, at any rate, was von Baer's opinion, who a.s.signs to Pander the glory of the discovery of the germ-layers. "You," he writes, "through your clearer recognition of the splitting of the germ--a process which remained dark to Wolff--have shed a light upon all forms of development" (p. xxi.).

We have now seen, following von Baer's exposition, how development is essentially a process of differentiation, a progress from the general to the special, from the h.o.m.ogeneous to the heterogeneous; we have a.n.a.lysed the process into its three subordinate processes--primary, histological and morphological differentiation. So far we have considered development in general and the laws which govern it; we have now to consider the varieties of development which the animal kingdom offers in such profusion, in order to discover what relations exist between them. This is the problem set in the fifth Scholion. Baer at once brings us face to face with the solution of the problem attempted in the Meckel-Serres law. It is a generally received opinion, he writes, that the higher animals repeat in their development the adult stages of the lower, and this is held to be the essential law governing the relation of the variety of development to the variety of adult form. This opinion arose when there was little real knowledge of embryology; it threw light indeed upon certain cases of monstrous development, but it was pushed altogether too far. It complicated itself with a belief in a historical evolution;--"People gradually learnt to think of the different animal forms as developed one from another--and seemed, in some circles at least, determined to forget that this metamorphosis could only be conceptual" (p. 200). At the same time the theory of parallelism led men to rehabilitate the outworn conception of the scale of beings, to maintain that animals form one single series of increasing complexity, a scale which the higher members must mount step by step in their development--from which it followed that evolution, whether conceived as an ideal or as an historical process, could take place only along one line, could be only progressive or regressive. Not all the supporters of the theory of parallelism held these extreme views, but conclusions of this kind were natural and logical enough.

Von Baer had soon found in the course of his embryological studies that the facts did not at all fit in with the doctrine of parallelism; the developing chick, for example, was at a very early stage demonstrably a Vertebrate, and did not recapitulate in its early stages the organisation of a polyp, a worm or a mollusc. He had published his doubts in 1823, but his final confutation of the theory of parallelism is found in this Scholion.

If it were true, he says, that the essential thing in the development of an animal is this repet.i.tion of lower organisations, then certain deductions could be drawn, which one would expect to find confirmed in Nature. The first deduction would be that no structures should appear in the embryo of the higher animals that are not found in the lower animals. But this is not confirmed by the facts--no adult among the lower animals, for instance, has a yolk-sac like that of the chick embryo. Again, if the law of parallelism were true, the mammalian embryo would have to repeat the organisation of, among other groups, insects and birds. But the embryo _in utero_ is surrounded by fluid and cannot possibly breathe free air, so it cannot possibly repeat the structure of either insects or birds, which are pre-eminently air-organisms.

Generally speaking, indeed, we find in all the higher embryos special structures which adapt them to the very special conditions of their development, and these we never find as permanent structures in the lower animals. The supporters of the theory of parallelism might, however, admit the existence of such special embryonic organs without greatly prejudicing their case, for these temporary organs stand to some extent outside the scope of the theory.

But they would have to face a second and more important deduction from their views, namely, that the higher animals should repeat at every stage of their development the whole organisation of some lower animal, and not merely agree with them in isolated details of structure. The deduction is, however, not borne out by the facts. The embryo of a mammal resembles in many points, at different stages of its development, the adult state of a fish; it has gill-slits and complete aortic arches, a two-chambered heart, and so on. But at no time does it combine all the essential characters of a fish; nor has it ever the tail of a fish, nor the fins, nor the shape. Any recapitulation there may be is a recapitulation of single organs, there is never a repet.i.tion of the complete organisation of a fish. This is indeed the fundamental criticism of the theory of parallelism; and if it applies even within the limits of the vertebrate phylum, so much the more does it apply to comparisons between embryonic Vertebrates and adult Invertebrates.

There are also some lesser arguments which might be urged against the theory of parallelism. If the theory were strictly true, no state which is permanent in a higher animal could be pa.s.sed through by an animal lower in the scale. But birds, which are lower in the scale than mammals, pa.s.s through a stage in which they resemble mammals in certain respects much more than they do when adult, for in an embryonic condition they agree with mammals in having no feathers, no air sacs, no pneumatic sacs in the bones, no beak. Their brain also resembles that of mammals more in an earlier stage than it does later. So, too, myriapods and hydrachnids have at birth three pairs of feet, and resemble at this stage adult insects, which form a higher cla.s.s.

Again, were the a.n.a.logy between the development of the individual and the evolution of the _ech.e.l.le des etres_ complete, organs and organ-systems ought to develop in the individual in the order in which they appear in the scale of beings. But this is not always the case. In fish the hinder extremity develops only its terminal joint, while in the embryos of higher animals the basal joint is the first to appear.

Another consequence one would expect to find realised, were the theory of parallelism correct, is the late appearance in development of parts which are confined to the higher animals. In the development of a Vertebrate accordingly one would not expect the vertebrae to appear before the embryo had pa.s.sed through many Invertebrate stages. But experience shows the direct contrary, for in the chick the rudiments of the vertebral axis appear sooner than any other part.

The theory of parallelism or recapitulation then is not borne out by the facts, and clearly cannot be the law which we are seeking. But what then is the true relation between the variety of development and the variety of adult structure? Before answering this question we must review the varied forms of adult organisation and consider in what relations they stand to one another. In particular we must enquire whether they belong to one type or to many. One point is here cardinal--we must distinguish between the _type_ of organisation and the _grade_ of differentiation.

By "type" von Baer means the structural plan of the organism. "I call the _type_ the spatial relationship of the organic elements and organs"

(p. 208). Each type of organisation characterises one of the big groups of animals; the lesser groups represent "grade" modifications of the type. "The product of the degree of differentiation and the type gives the several great groups of animals which are called cla.s.ses" (p. 208).

_Ausbildung_ (differentiation) takes place in one or other of several directions, in adaptation, for instance, to life in the water or to life in the air.

There are, von Baer considers, four main types--(1) the peripheral or radiate type, (2) the longitudinal type, (3) the ma.s.sive or molluscan type, (4) the vertebrate type. The radiate type is shown by discoid infusoria, by medusae, by starfish and their allies. The longitudinal type characterises such genera as _Vibrio_, _Filaria_, _Gordius_, and all the annulate animals. Mollusca, rotifers, polyzoa, and such infusoria as are not included in types (1) and (2) belong to the ma.s.sive type, in which the body and its parts form rounded ma.s.ses. The longitudinal type is predominantly "animal," the ma.s.sive type predominantly "plastic" (vegetative). The vertebrate type has both the "animal" and the "plastic" organs highly developed. In the symmetrical arrangement of the animal parts it resembles the longitudinal type; its plastic parts with their asymmetrical arrangement and rounded shape belong to the ma.s.sive type.

These types of von Baer inevitably recall the "Embranchements" of Cuvier, with which they more or less coincide. It seems that von Baer arrived at his types (from the study of adult structure) independently of Cuvier, though the priority of publication rests with Cuvier.[174]

Now it is clear that the development of the individual, which is essentially an _Ausbildung_, a differentiation, is directly comparable with the grade-differentiation of forms within the type. And just as the type rules all its varied modifications, so does the development of the individual take place always within the bounds imposed by type. This is von Baer's chief contribution to the theory of embryonic relationships--the law that "the type of organisation determines the manner of development" (p. xxii.). Development is not merely from the general to the special--there are at least four distinct "general"

types, from which the special is developed. The type is fixed in the very earliest stages of development--the embryo of a Vertebrate is from the very beginning a Vertebrate (p. 220), and it shows at no time any agreement in total organisation with any Invertebrate. The types are independent of one another; differentiation and development follow a different course in each of them. Not but what some a.n.a.logies can be found between the very earliest stages of embryos of different type.

Thus vertebrate and annulate embryos agree in certain points at the time of the formation of the primitive streak. And in the earliest stage of all, the egg-stage, there is probably agreement between all the types.

In eggs with yolk, whether vertebrate or annulate, there is always a separation into an animal and a plastic layer. It seems, too, as if a hollow sphere were a constant stage in the development of all animals (pp. 224, 258). Apart from these a.n.a.logies, development takes an entirely independent course in each of the four main types, and no embryo of one of the higher types repeats in its development the peculiar organisation of any adult of the lower types.

If we consider now development within the type, which is the only legitimate thing to do, we arrive at certain laws governing the relation of embryos to one another. For instance, at a certain stage vertebrate embryos are uncommonly alike. Von Baer had two in spirit which he was unable to a.s.sign to their cla.s.s among amniotes; they might have been lizard, bird, or mammal, he could not say definitely which.[175] Generally the farther back we go in the development of Vertebrates the more alike we find the embryos. The type-characters are first to appear, then the cla.s.s characters, then the characters distinguishing the lesser cla.s.sificatory groups. "From a more general type the special gradually emerges" (p. 221). The chick is first a Vertebrate, then a land-vertebrate, then a bird, then a land-bird, then a gallinaceous bird, and finally _Gallus domesticus_. Development within the type is a progress from the general to the special, a real evolution. The more divergent two adults are, the farther back we must go in their development to find an agreement between their embryos. We can sum up the case in the following laws:--

"(1) _That the general characters of the big group to which the embryo belongs appear in development earlier than the special characters._ In agreement with this is the fact that the vesicular form is the most general form of all; for what is common in a greater degree to all animals than the opposition of an internal and an external surface?

"(2) _The less general structural relations are formed after the more general, and so on until the most special appear._

"(3) _The embryo of any given form, instead of pa.s.sing through the state of other definite forms, on the contrary separates itself from them._

"(4) _Fundamentally the embryo of a higher animal form never resembles the adult of another animal form, but only its embryo_" (p. 224).

These laws relating to development within the limits of type are destructive of even a limited application of the theory of parallelism, for not even within the limits of the type is there a real scale which the higher forms must mount; each embryo develops for itself, and diverges sooner or later from the embryos of other species, the divergence coming earlier the greater the difference between the adult forms. It is only because the lower less-differentiated adult forms happen to be little divergent from the generalised or embryonic type, that they show a certain similarity with the embryos of the higher more differentiated members of the group. Such similarity, however, is due to no necessary law governing the development of the higher animals; it is, on the contrary, merely a consequence of the organisation of these lower animals (p. 224).

Von Baer goes on to show what are the distinguishing embryological characters of the types and cla.s.ses, working out a dichotomous schema of development, which each embryo must follow, branching off early or late to its terminal point, according to the lower or higher goal it has to reach.

One important consequence for morphology results from von Baer's laws of differentiation within the type. If the embryo develops from the general to the special, then the state in which each organ or organ-system first appears must represent the general or typical state of that organ within the group. Embryology will therefore be of great a.s.sistance to comparative anatomy, whose chief aim it is to discover the generalised type, the common plan of structure, upon which the animals of each big group are built. And the surest way to determine the true h.o.m.ologies of parts will be to study their early development. "For since each organ becomes what it is only through the manner of its development, its true value can be recognised only from its method of formation. At present, we form our judgments by an undefined intuition, instead of regarding each organ merely as an isolated product of its fundamental organ, and discerning from this standpoint the correspondences and dissimilarities in the different types" (p. 233). Parts, therefore, which develop from the same "fundamental organ," and in the last resort from the same germ-layer, have a certain kinship, which may even reach the degree of exact h.o.m.ology.

Now since the mode of development in each type is peculiar to that type, organs of the same name in different types must not necessarily be accounted h.o.m.ologous, even if they correspond exactly with one another in their general _functional_ relations to the rest of the organs. Thus the central nervous system of Arthropods must not be h.o.m.ologised with the central nervous system of Vertebrates, for it develops in a different manner. So, too, the brain of Arthropods or of Mollusca is not strictly comparable with the brain of Vertebrates. Again, the air-tubes or tracheae of insects are, like the trachea and bronchi of many Vertebrates, air-breathing organs. But the two organs are not h.o.m.ologous, for the air-tubes of Vertebrates are developed from the alimentary tube ("fundamental organ" of the alimentary system, developed from the vegetative layer), while the air-tubes of insects arise either by histological differentiation, or by inv.a.g.i.n.ation of the skin (p.

236). Organs can be h.o.m.ologous only within the limits of the big groups; there can be no question of h.o.m.ology between members of different types.

The development of plants, like the development of animals, is essentially a progress from the general to the special (p. 242).

Botanists have not been troubled by any recapitulation theory, and in founding their big groups, Acotyledons, Monocotyledons, and Dicotyledons, upon embryological characters, they were guided by true principles, which ought indeed to be followed in zoology. If we knew the development of all kinds of animals sufficiently well, then the best way to cla.s.sify them would be according to the characters they show in their early development, for it is in early development that they show the characters of the type in their most generalised form. As it is, we have in our ignorance to establish the big groups by the study of adult structure, but we find, on putting together all we know of comparative embryology, that a cla.s.sification of animals according to the mode of their development gives, as is only natural, the same four groups as does the study of adult structure. The four types of development are thus:--

(1) The double-symmetrical, which is found in Vertebrates. It is called the double-symmetrical, because in Vertebrates development takes place from a central axis (notochord) in two directions, upwards and downwards, in such a way that two tubes are formed, one above and one below the axis. (2) The second type is the symmetrical, which is shown by Annulates. A primitive streak is formed on the ventral surface of the yolk; development proceeds symmetrically on both sides of the streak.

(3) Radiate development is probably typical of the radiate structural type. (4) In the ma.s.sive type, the development seems to be a spiral one.

Common to most modes is a separation of the germ into animal and plastic layers, a separation which seems to be conditioned largely by the presence of yolk. A cla.s.sification based upon embryological characters ought to be applied even to the lesser groups and would here prove itself of service. Embryology, for instance, fully supports de Blainville's separation of Batrachia from true reptiles,[176] for reptiles develop an amnion and Batrachia do not.

We come now to the sixth and last Scholion. Development is a true evolution of the special from the general, so runs von Baer's most general law of all. This can be expressed in a slightly different way, and the words which he chooses in the sixth Scholion to express this final and most general result are these:--"The developmental history of the individual is the history of the growing individuality in every respect" (p. 263). The greatest modern treatise on embryology ends on a splendid note. One creative thought rules all the forms of life. And more--"It is this same thought that in cosmic s.p.a.ce gathered the scattered ma.s.ses into spheres and bound them together in the solar system, the same that from the weathered dust on the surface of the metallic planets brought forth the forms of life. And this thought is nought else but life itself, and the words and syllables in which life expresses itself are the varied forms of the living" (p. 264).

Von Baer reminds one greatly of Cuvier. There is the same sheer intellectual power, the same sanity of mind, the same synthetic grip.

Von Baer, like Cuvier, never forgot that he was working with living things; he was saturated, like Cuvier, with the sense of their functional adaptedness. In his paper on the external and internal skeleton[177] he gives a masterly a.n.a.lysis of the functional modifications of the limbs in Vertebrates, and the whole paper indeed, with its sober attack on transcendentalism, is a vindication as much of the functional point of view as of the importance of embryology.

Both Cuvier and von Baer, by the very sanity of their views, found themselves in partial opposition to the theories current in their time.