In the ova which undergo this superficial cleavage the formative yelk is sharply divided from the nutritive yelk, as in the preceding cases of the ova of birds, reptiles, fishes, etc.; the formative yelk alone undergoes cleavage. But while in the ova with discoid gastrulation the formative yelk is not in the centre, but at one pole of the uni-axial ovum, and the food-yelk gathered at the other pole, in the ova with superficial cleavage we find the formative yelk spread over the whole surface of the ovum; it encloses spherically the food-yelk, which is acc.u.mulated in the middle of the ova. As the segmentation only affects the former and not the latter, it is bound to be entirely "superficial"; the store of food in the middle is quite untouched by it. As a rule, it proceeds in regular geometrical progression. In the end the whole of the formative yelk divides into a number of small and h.o.m.ogeneous cells, which lie close together in a single stratum on the entire surface of the ovum, and form a superficial blastoderm. This blastoderm is a simple, completely closed vesicle, the internal cavity of which is entirely full of food-yelk. This real blastula only differs from that of the primitive ova in its chemical composition. In the latter the content is water or a watery jelly; in the former it is a thick mixture, rich in food-yelk, of alb.u.minous and fatty substances. As this quant.i.ty of food-yelk fills the centre of the ovum before cleavage begins, there is no difference in this respect between the morula and the blastula. The two stages rather agree in this.

When the blastula is fully formed, we have again in this case the important folding or inv.a.g.i.n.ation that determines gastrulation. The s.p.a.ce between the skin-layer and the gut-layer (the remainder of the segmentation-cavity) remains full of food-yelk, which is gradually used up. This is the only material difference between our vesicular gastrula (perigastrula) and the original form of the bell-gastrula (archigastrula). Clearly the one has been developed from the other in the course of time, owing to the acc.u.mulation of food-yelk in the centre of the ovum.* (* On the reduction of all forms of gastrulation to the original palingenetic form see especially the lucid treatment of the subject in Arnold Lang's Manual of Comparative Anatomy (1888), Part 1.)

We must count it an important advance that we are thus in a position to reduce all the various embryonic phenomena in the different groups of animals to these four princ.i.p.al forms of segmentation and gastrulation. Of these four forms we must regard one only as the original palingenetic, and the other three as cenogenetic and derivative. The unequal, the discoid, and the superficial segmentation have all clearly arisen by secondary adaptation from the primary segmentation; and the chief cause of their development has been the gradual formation of the food-yelk, and the increasing ant.i.thesis between animal and vegetal halves of the ovum, or between ectoderm (skin-layer) and entoderm (gut-layer).

(FIGURE 1.72. Gastrula of the placental mammal (epigastrula from the rabbit), longitudinal section through the axis. e ectodermic cells (sixty-four, lighter and smaller), i entodermic cells (thirty-two, darker and larger), d central entodermic cell, filling the primitive gut-cavity, o peripheral entodermic cell, stopping up the opening of the primitive mouth (yelk-stopper in the Rusconian a.n.u.s).)

(FIGURE 1.73. Gastrula of the rabbit. A as a solid, spherical cl.u.s.ter of cells, B changing into the embryonic vesicle, bp primitive mouth, ep ectoderm, hy entoderm.)

The numbers of careful studies of animal gastrulation that have been made in the last few decades have completely established the views I have expounded, and which I first advanced in the years 1872 to 1876.

For a time they were greatly disputed by many embryologists. Some said that the original embryonic form of the metazoa was not the gastrula, but the "planula"--a double-walled vesicle with closed cavity and without mouth-aperture; the latter was supposed to pierce through gradually. It was afterwards shown that this planula (found in several sponges, etc.) was a later evolution from the gastrula. It was also shown that what is called delamination--the rise of the two primary germinal layers by the folding of the surface of the blastoderm (for instance, in the Geryonidae and other medusae)--was a secondary formation, due to cenogenetic variations from the original inv.a.g.i.n.ation of the blastula. The same may be said of what is called "immigration," in which certain cells or groups of cells are detached from the simple layer of the blastoderm, and travel into the interior of the blastula; they attach themselves to the inner wall of the blastula, and form a second internal epithelial layer--that is to say, the entoderm. In these and many other controversies of modern embryology the first requisite for clear and natural explanation is a careful and discriminative distinction between palingenetic (hereditary) and cenogenetic (adaptive) processes. If this is properly attended to, we find evidence everywhere of the biogenetic law.

CHAPTER 1.10. THE COELOM THEORY.

The two "primary germinal layers" which the gastraea theory has shown to be the first foundation in the construction of the body are found in this simplest form throughout life only in animals of the lowest grade--in the gastraeads, olynthus (the stem-form of the sponges), hydra, and similar very simple animals. In all the other animals new strata of cells are formed subsequently between these two primary body-layers, and these are generally comprehended under the t.i.tle of the middle layer, or mesoderm. As a rule, the various products of this middle layer afterwards const.i.tute the great bulk of the animal frame, while the original entoderm, or internal germinal layer, is restricted to the clothing of the alimentary ca.n.a.l and its glandular appendages; and, on the other hand, the ectoderm, or external germinal layer, furnishes the outer clothing of the body, the skin and nervous system.

In some large groups of the lower animals, such as the sponges, corals, and flat-worms, the middle germinal layer remains a single connected ma.s.s, and most of the body is developed from it; these have been called the three-layered metazoa, in opposition to the two-layered animals described. Like the two-layered animals, they have no body-cavity--that is to say, no cavity distinct from the alimentary system. On the other hand, all the higher animals have this real body-cavity (coeloma), and so are called coelomaria. In all these we can distinguish four secondary germinal layers, which develop from the two primary layers. To the same cla.s.s belong all true vermalia (excepting the platodes), and also the higher typical animal stems that have been evolved from them--molluscs, echinoderms, articulates, tunicates, and vertebrates.

(FIGURES 1.74 AND 1.75. Diagram of the four secondary germinal layers, transverse section through the metazoic embryo: Figure 1.74 of an annelid, Figure 1.75 of a vermalian. a primitive gut, dd ventral glandular layer, df ventral fibre-layer, hm skin-fibre-layer, hs skin-sense-layer, u beginning of the rudimentary kidneys, n beginning of the nerve-plates.)

The body-cavity (coeloma) is therefore a new acquisition of the animal body, much younger than the alimentary system, and of great importance. I first pointed out this fundamental significance of the coelom in my Monograph on the Sponges (1872), in the section which draws a distinction between the body-cavity and the gut-cavity, and which follows immediately on the germ-layer theory and the ancestral tree of the animal kingdom (the first sketch of the gastraea theory).

Up to that time these two princ.i.p.al cavities of the animal body had been confused, or very imperfectly distinguished; chiefly because Leuckart, the founder of the coelenterata group (1848), has attributed a body-cavity, but not a gut-cavity, to these lowest metazoa. In reality, the truth is just the other way about.

The ventral cavity, the original organ of nutrition in the multicellular animal-body, is the oldest and most important organ of all the metazoa, and, together with the primitive mouth, is formed in every case in the gastrula as the primitive gut; it is only at a much later stage that the body-cavity, which is entirely wanting in the coelenterata, is developed in some of the metazoa between the ventral and the body wall. The two cavities are entirely different in content and purport. The alimentary cavity (enteron) serves the purpose of digestion; it contains water and food taken from without, as well as the pulp (chymus) formed from this by digestion. On the other hand, the body-cavity, quite distinct from the gut and closed externally, has nothing to do with digestion; it encloses the gut itself and its glandular appendages, and also contains the s.e.xual products and a certain amount of blood or lymph, a fluid that is transuded through the ventral wall.

As soon as the body-cavity appears, the ventral wall is found to be separated from the enclosing body-wall, but the two continue to be directly connected at various points. We can also then always distinguish a number of different layers of tissue in both walls--at least two in each. These tissue-layers are formed originally from four different simple cell-layers, which are the much-discussed four secondary germinal layers. The outermost of these, the skin-sense-layer (Figures 1.74 and 1.75 hs), and the innermost, the gut-gland-layer (dd), remain at first simple epithelia or covering-layers. The one covers the outer surface of the body, the other the inner surface of the ventral wall; hence they are called confining or limiting layers. Between them are the two middle-layers, or mesoblasts, which enclose the body-cavity.

(FIGURE 1.76. Coelomula of sagitta (gastrula with a couple of coelom-pouches. (From Kowalevsky.) bl.p primitive mouth, al primitive gut, pv coelom-folds, m permanent mouth.)

The four secondary germinal layers are so distributed in the structure of the body in all the coelomaria (or all metazoa that have a body-cavity) that the outer two, joined fast together, const.i.tute the body-wall, and the inner two the ventral wall; the two walls are separated by the cavity of the coelom. Each of the walls is made up of a limiting layer and a middle layer. The two limiting layers chiefly give rise to epithelia, or covering-tissues, and glands and nerves, while the middle layers form the great bulk of the fibrous tissue, muscles, and connective matter. Hence the latter have also been called fibrous or muscular layers. The outer middle layer, which lies on the inner side of the skin-sense-layer, is the skin fibre-layer; the inner middle layer, which attaches from without to the ventral glandular layer, is the ventral fibre layer. The former is usually called briefly the parietal, and the latter the visceral layer or mesoderm.

Of the many different names that have been given to the four secondary germinal layers, the following are those most in use to-day:--

1. Skin-sense-layer (outer limiting layer) and 2. Skin-fibre-layer (outer middle layer).

I. Neural layer (neuroblast) and II. Parietal layer (myoblast). The two secondary germinal layers of the body-wall: 1. Epithelial. 2.

Fibrous.

3. Gut-fibre-layer (inner middle layer) and 4. Gut-gland-layer (inner limiting layer).

III. Visceral layer (gon.o.blast) and IV. Enteral layer (enteroblast).

The two secondary germinal layers of the gut-wall: 3. Fibrous. 4.

Epithelial.

The first scientist to recognise and clearly distinguish the four secondary germinal layers was Baer. It is true that he was not quite clear as to their origin and further significance, and made several mistakes in detail in explaining them. But, on the whole, their great importance did not escape him. However, in later years his view had to be given up in consequence of more accurate observations. Remak then propounded a three-layer theory, which was generally accepted. These theories of cleavage, however, began to give way thirty years ago, when Kowalevsky (1871) showed that in the case of Sagitta (a very clear and typical subject of gastrulation) the two middle germinal layers and the two limiting layers arise not by cleavage, but by folding--by a secondary inv.a.g.i.n.ation of the primary inner germ-layer.

This inv.a.g.i.n.ation or folding proceeds from the primitive mouth, at the two sides of which (right and left) a couple of pouches are formed. As these coelom-pouches or coelom-sacs detach themselves from the primitive gut, a double body-cavity is formed (Figures 1.74 to 1.76).

(FIGURE 1.77. Coelomula of sagitta, in section. (From Hertwig.) D dorsal side, V ventral side, ik inner germinal layer, mv visceral mesoblast, lh body-cavity, mp parietal mesoblast, ak outer germinal layer.)

The same kind of coelom-formation as in sagitta was afterwards found by Kowalevsky in brachiopods and other invertebrates, and in the lowest vertebrate--the amphioxus. Further instances were discovered by two English embryologists, to whom we owe very considerable advance in ontogeny--E. Ray-Lankester and F. Balfour. On the strength of these and other studies, as well as most extensive research of their own, the brothers Oscar and Richard Hertwig constructed in 1881 the Coelom Theory. In order to appreciate fully the great merit of this illuminating and helpful theory, one must remember what a chaos of contradictory views was then represented by the "problem of the mesoderm," or the much-disputed "question of the origin of the middle germinal layer." The coelom theory brought some light and order into this infinite confusion by establishing the following points: 1. The body-cavity originates in the great majority of animals (especially in all the vertebrates) in the same way as in sagitta: a couple of pouches or sacs are formed by folding inwards at the primitive mouth, between the two primary germinal layers; as these pouches detach from the primitive gut, a pair of coelom-sacs (right and left) are formed; the coalescence of these produces a simple body-cavity. 2. When these coelom-embryos develop, not as a pair of hollow pouches, but as solid layers of cells (in the shape of a pair of mesodermal streaks)--as happens in the higher vertebrates--we have a secondary (cenogenetic) modification of the primary (palingenetic) structure; the two walls of the pouches, inner and outer, have been pressed together by the expansion of the large food-yelk. 3. Hence the mesoderm consists from the first of TWO genetically distinct layers, which do not originate by the cleavage of a primary simple middle layer (as Remak supposed).

4. These two middle layers have, in all vertebrates, and the great majority of the invertebrates, the same radical significance for the construction of the animal body; the inner middle layer, or the visceral mesoderm, (gut-fibre layer), attaches itself to the original entoderm, and forms the fibrous, muscular, and connective part of the visceral wall; the outer middle layer, or the parietal mesoderm (skin-fibre-layer), attaches itself to the original ectoderm and forms the fibrous, muscular, and connective part of the body-wall. 5. It is only at the point of origination, the primitive mouth and its vicinity, that the four secondary germinal layers are directly connected; from this point the two middle layers advance forward separately between the two primary germinal layers, to which they severally attach themselves. 6. The further separation or differentiation of the four secondary germinal layers and their division into the various tissues and organs take place especially in the later fore-part or head of the embryo, and extend backwards from there towards the primitive mouth.

(FIGURE 1.78. Section of a young sagitta. (From Hertwig.) dh visceral cavity, ik and ak inner and outer limiting layers, mv and mp inner and outer middle layers, lk body-cavity, dm and vm dorsal and visceral mesentery.)

All animals in which the body-cavity demonstrably arises in this way from the primitive gut (vertebrates, tunicates, echinoderms, articulates, and a part of the vermalia) were comprised by the Hertwigs under the t.i.tle of enterocoela, and were contrasted with the other groups of the pseudocoela (with false body-cavity) and the coelenterata (with no body-cavity). However, this radical distinction and the views as to cla.s.sification which it occasioned have been shown to be untenable. Further, the absolute differences in tissue-formation which the Hertwigs set up between the enterocoela and pseudocoela cannot be sustained in this connection. For these and other reasons their coelom-theory has been much criticised and partly abandoned.

Nevertheless, it has rendered a great and lasting service in the solution of the difficult problem of the mesoderm, and a material part of it will certainly be retained. I consider it an especial merit of the theory that it has established the ident.i.ty of the development of the two middle layers in all the vertebrates, and has traced them as cenogenetic modifications back to the original palingenetic form of development that we still find in the amphioxus. Carl Rabl comes to the same conclusion in his able Theory of the Mesoderm, and so do Ray-Lankester, Rauber, Kupffer, Ruckert, Selenka, Hatschek, and others. There is a general agreement in these and many other recent writers that all the different forms of coelom-construction, like those of gastrulation, follow one and the same strict hereditary law in the vast vertebrate stem; in spite of their apparent differences, they are all only cenogenetic modifications of one palingenetic type, and this original type has been preserved for us down to the present day by the invaluable amphioxus.

(FIGURES 1.79 AND 1.80. Transverse section of amphioxus-larvae. (From Hatschek.) Figure 1.79 at the commencement of coelom formation (still without segments), Figure 1.80 at the stage with four primitive segments. ak, ik, mk outer, inner, and middle germinal layer, hp horn plate, mp medullary plate, ch chorda, asterisk and asterisk, disposition of the coelom-pouches, lh body-cavity.)

But before we go into the regular coelomation of the amphioxus, we will glance at that of the arrow-worm (Sagitta), a remarkable deep-sea worm that is interesting in many ways for comparative anatomy and ontogeny. On the one hand, the transparency of the body and the embryo, and, on the other hand, the typical simplicity of its embryonic development, make the sagitta a most instructive object in connection with various problems. The cla.s.s of the chaetogatha, which is only represented by the cognate genera of Sagitta and Spadella, is in another respect also a most remarkable branch of the extensive vermalia stem. It was therefore very gratifying that Oscar Hertwig (1880) fully explained the anatomy, cla.s.sification, and evolution of the chaetognatha in his careful monograph.

The spherical blastula that arises from the impregnated ovum of the sagitta is converted by a folding at one pole into a typical archigastrula, entirely similar to that of the Monoxenia which I described (Chapter 1.8, Figure 1.29). This oval, uni-axial cup-larva (circular in section) becomes bilateral (or tri-axial) by the growth of a couple of coelom-pouches from the primitive gut (Figures 1.76 and 1.77). To the right and left a sac-shaped fold appears towards the top pole (where the permanent mouth, m, afterwards arises). The two sacs are at first separated by a couple of folds of the entoderm (Figure 1.76 pv), and are still connected with the primitive gut by wide apertures; they also communicate for a short time with the dorsal side (Figure 1.77 d). Soon, however, the coelom-pouches completely separate from each other and from the primitive gut; at the same time they enlarge so much that they close round the primitive gut (Figure 1.78).

But in the middle line of the dorsal and ventral sides the pouches remain separated, their approaching walls joining here to form a thin vertical part.i.tion, the mesentery (dm and vm). Thus Sagitta has throughout life a double body-cavity (Figure 1.78 lk), and the gut is fastened to the body-wall both above and below by a mesentery--below by the ventral mesentery (vm), and above by the dorsal mesentery (dm).

The inner layer of the two coelom-pouches (mv) attaches itself to the entoderm (ik), and forms with it the visceral wall. The outer layer (mp) attaches itself to the ectoderm (ak), and forms with it the outer body-wall. Thus we have in Sagitta a perfectly clear and simple ill.u.s.tration of the original coelomation of the enterocoela. This palingenetic fact is the more important, as the greater part of the two body-cavities in Sagitta changes afterwards into s.e.xual glands--the fore or female part into a pair of ovaries, and the hind or male part into a pair of t.e.s.t.i.c.l.es.

Coelomation takes place with equal clearness and transparency in the case of the amphioxus, the lowest vertebrate, and its nearest relatives, the invertebrate tunicates, the sea-squirts. However, in these two stems, which we cla.s.s together as Chordonia, this important process is more complex, as two other processes are a.s.sociated with it--the development of the chorda from the entoderm and the separation of the medullary plate or nervous centre from the ectoderm. Here again the skulless amphioxus has preserved to our own time by tenacious heredity the chief phenomena in their original form, while it has been more or less modified by embryonic adaptation in all the other vertebrates (with skulls). Hence we must once more thoroughly understand the palingenetic embryonic features of the lancelet before we go on to consider the cenogenetic forms of the craniota.

(FIGURES 1.81 AND 1.82. Transverse section of amphioxus embryo. Figure 1.81 at the stage with five somites, Figure 1.82 at the stage with eleven somites. (From Hatschek.) ak outer germinal layer, mp medullary plate, n nerve-tube, ik inner germinal layer, dh visceral cavity, lh body-cavity, mk middle germinal layer (mk1 parietal, mk2 visceral), us primitive segment, ch chorda.)

The coelomation of the amphioxus, which was first observed by Kowalevsky in 1867, has been very carefully studied since by Hatschek (1881). According to him, there are first formed on the bilateral gastrula we have already considered (Figures 1.36 and 1.37) three parallel longitudinal folds--one single ectodermal fold in the central line of the dorsal surface, and a pair of entodermic folds at the two sides of the former. The broad ectodermal fold that first appears in the middle line of the flattened dorsal surface, and forms a shallow longitudinal groove, is the beginning of the central nervous system, the medullary tube. Thus the primary outer germinal layer divides into two parts, the middle medullary plate (Figure 1.81 mp) and the h.o.r.n.y-plate (ak), the beginning of the outer skin or epidermis. As the parallel borders of the concave medullary plate fold towards each other and grow underneath the h.o.r.n.y-plate, a cylindrical tube is formed, the medullary tube (Figure 1.82 n); this quickly detaches itself altogether from the h.o.r.n.y-plate. At each side of the medullary tube, between it and the alimentary tube (Figures 1.79 to 1.82 dh), the two parallel longitudinal folds grow out of the dorsal wall of the alimentary tube, and these form the two coelom-pouches (Figures 1.80 and 1.81 lh). This part of the entoderm, which thus represents the first structure of the middle germinal layer, is shown darker than the rest of the inner germinal layer in Figures 1.79 to 1.82. The edges of the folds meet, and thus form closed tubes (Figure 1.81 in section).

During this interesting process the outline of a third very important organ, the chorda or axial rod, is being formed between the two coelom-pouches. This first foundation of the skeleton, a solid cylindrical cartilaginous rod, is formed in the middle line of the dorsal primitive gut-wall, from the entodermal cell-streak that remains here between the two coelom-pouches (Figures 1.79 to 1.82 ch).

The chorda appears at first in the shape of a flat longitudinal fold or a shallow groove (Figures 1.80 and 1.81); it does not become a solid cylindrical cord until after separation from the primitive gut (Figure 1.82). Hence we might say that the dorsal wall of the primitive gut forms three parallel longitudinal folds at this important period--one single fold and a pair of folds. The single middle fold becomes the chorda, and lies immediately below the groove of the ectoderm, which becomes the medullary tube; the pair of folds to the right and left lie at the sides between the former and the latter, and form the coelom-pouches. The part of the primitive gut that remains after the cutting off of these three dorsal primitive organs is the permanent gut; its entoderm is the gut-gland-layer or enteric layer.

(FIGURES 1.83 AND 1.84. Chordula of the amphioxus. Figure 1.83 median longitudinal section (seen from the left). Figure 1.84 transverse section. (From Hatschek.) In Figure 1.83 the coelom-pouches are omitted, in order to show the chordula more clearly. Figure 1.84 is rather diagrammatic. h h.o.r.n.y-plate, m medullary tube, n wall of same (n apostrophe, dorsal, n double apostrophe, ventral), ch chorda, np neuroporus, ne ca.n.a.lis neurentericus, d gut-cavity, r gut dorsal wall, b gut ventral wall, z yelk-cells in the latter, u primitive mouth, o mouth-pit, p promesoblasts (primitive or polar cells of the mesoderm), w parietal layer, v visceral layer of the mesoderm, c coelom, f rest of the segmentation-cavity.

FIGURES 1.85 AND 1.86. Chordula of the amphibia (the ringed adder).

(From Goette.) Figure 85 median longitudinal section (seen from the left), Figure 1.86 transverse section (slightly diagrammatic).

Lettering as in Figures 1.83 and 1.84.

FIGURES 1.87 AND 1.88. Diagrammatic vertical section of coelomula-embryos of vertebrates. (From Hertwig.) Figure 1.87, vertical section THROUGH the primitive mouth, Figure 1.88, vertical section BEFORE the primitive mouth. u primitive mouth, ud primitive gut. d yelk, dk yelk-nuclei, dh gut-cavity, lh body-cavity, mp medullary plate, ch chorda plate, ak and ik outer and inner germinal layers, pb parietal and vb visceral mesoblast.

FIGURES 1.89 AND 1.90. Transverse section of coelomula embryos of triton. (From Hertwig.) Figure 1.89, section THROUGH the primitive mouth. Figure 1.90, section in front of the primitive mouth, u primitive mouth. dh gut-cavity, dz yelk-cells, dp yelk-stopper, ak outer and ik inner germinal layer, pb parietal and vb visceral middle layer, m medullary plate, ch chorda.)

I give the name of chordula or chorda-larva to the embryonic stage of the vertebrate organism which is represented by the amphioxus larva at this period (Figures 1.83 and 1.84, in the third period of development according to Hatschek). (Strabo and Plinius give the name of cordula or cordyla to young fish larvae.) I ascribe the utmost phylogenetic significance to it, as it is found in all the chorda-animals (tunicates as well as vertebrates) in essentially the same form.

Although the acc.u.mulation of food-yelk greatly modifies the form of the chordula in the higher vertebrates, it remains the same in its main features throughout. In all cases the nerve-tube (m) lies on the dorsal side of the bilateral, worm-like body, the gut-tube (d) on the ventral side, the chorda (ch) between the two, on the long axis, and the coelom pouches (c) at each side. In every case these primitive organs develop in the same way from the germinal layers, and the same organs always arise from them in the mature chorda-animal. Hence we may conclude, according to the laws of the theory of descent, that all these chordonia or chordata (tunicates and vertebrates) descend from an ancient common ancestral form, which we may call Chordaea. We should regard this long-extinct Chordaea, if it were still in existence, as a special cla.s.s of unarticulated worm (chordaria). It is especially noteworthy that neither the dorsal nerve-tube nor the ventral gut-tube, nor even the chorda that lies between them, shows any trace of articulation or segmentation; even the two coelom-sacs are not segmented at first (though in the amphioxus they quickly divide into a series of parts by transverse folding). These ontogenetic facts are of the greatest importance for the purpose of learning those ancestral forms of the vertebrates which we have to seek in the group of the unarticulated vermalia. The coelom-pouches were originally s.e.xual glands in these ancient chordonia.

(FIGURE 1.91. A, B, C. Vertical section of the dorsal part of three triton-embryos. (From Hertwig.) In Figure A the medullary swellings (the parallel borders of the medullary plate) begin to rise; in Figure B they grow towards each other; in Figure C they join and form the medullary tube. mp medullary plate, mf medullary folds, n nerve-tube, ch chorda, lh body-cavity, mk1 and mk2 parietal and visceral mesoblasts, uv primitive-segment cavities, ak ectoderm, ik entoderm, dz yelk-cells, dh gut-cavity.)

From the evolutionary point of view the coelom-pouches are, in any case, older than the chorda; since they also develop in the same way as in the chordonia in a number of invertebrates which have no chorda (for instance, Sagitta, Figures 1.76 to 1.78). Moreover, in the amphioxus the first outline of the chorda appears later than that of the coelom-sacs. Hence we must, according to the biogenetic law, postulate a special intermediate form between the gastrula and the chordula, which we will call coelomula, an unarticulated, worm-like body with primitive gut, primitive mouth, and a double body-cavity, but no chorda. This embryonic form, the bilateral coelomula (Figure 1.81), may in turn be regarded as the ontogenetic reproduction (maintained by heredity) of an ancient ancestral form of the coelomaria, the Coelomaea (cf. Chapter 2.20).

In Sagitta and other worm-like animals the two coelom-pouches (presumably gonads or s.e.x-glands) are separated by a complete median part.i.tion, the dorsal and ventral mesentery (Figure 1.78 dm and vm); but in the vertebrates only the upper part of this vertical part.i.tion is maintained, and forms the dorsal mesentery. This mesentery afterwards takes the form of a thin membrane, which fastens the visceral tube to the chorda (or the vertebral column). At the under side of the visceral tube the coelom-sacs blend together, their inner or median walls breaking down and disappearing. The body-cavity then forms a single simple hollow, in which the gut is quite free, or only attached to the dorsal wall by means of the mesentery.

The development of the body-cavity and the formation of the chordula in the higher vertebrates is, like that of the gastrula, chiefly modified by the pressure of the food-yelk on the embryonic structures, which forces its hinder part into a discoid expansion. These cenogenetic modifications seem to be so great that until twenty years ago these important processes were totally misunderstood. It was generally believed that the body-cavity in man and the higher vertebrates was due to the division of a simple middle layer, and that the latter arose by cleavage from one or both of the primary germinal layers. The truth was brought to light at last by the comparative embryological research of the Hertwigs. They showed in their Coelom Theory (1881) that all vertebrates are true enterocoela, and that in every case a pair of coelom-pouches are developed from the primitive gut by folding. The cenogenetic chordula-forms of the craniotes must therefore be derived from the palingenetic embryology of the amphioxus in the same way as I had previously proved for their gastrula-forms.

The chief difference between the coelomation of the acrania (amphioxus) and the other vertebrates (with skulls--craniotes) is that the two coelom-folds of the primitive gut in the former are from the first hollow vesicles, filled with fluid, but in the latter are empty pouches, the layers of which (inner and outer) close with each other.

In common parlance we still call a pouch or pocket by that name, whether it is full or empty. It is different in ontogeny; in some of our embryological literature ordinary logic does not count for very much. In many of the manuals and large treatises on this science it is proved that vesicles, pouches, or sacs deserve that name only when they are inflated and filled with a clear fluid. When they are not so filled (for instance, when the primitive gut of the gastrula is filled with yelk, or when the walls of the empty coelom-pouches are pressed together), these vesicles must not be cavities any longer, but "solid structures."

The acc.u.mulation of food-yelk in the ventral wall of the primitive gut (Figures 1.85 and 1.86) is the simple cause that converts the sac-shaped coelom-pouches of the acrania into the leaf-shaped coelom-streaks of the craniotes. To convince ourselves of this we need only compare, with Hertwig, the palingenetic coelomula of the amphioxus (Figures 1.80 and 1.81) with the corresponding cenogenetic form of the amphibia (Figures 1.89 to 1.90), and construct the simple diagram that connects the two (Figures 1.87 and 1.88). If we imagine the ventral half of the primitive gut-wall in the amphioxus embryo (Figures 1.79 to 1.84) distended with food-yelk, the vesicular coelom-pouches (lh) must be pressed together by this, and forced to extend in the shape of a thin double plate between the gut-wall and body-wall (Figures 1.86 and 1.87). This expansion follows a downward and forward direction. They are not directly connected with these two walls. The real unbroken connection between the two middle layers and the primary germ-layers is found right at the back, in the region of the primitive mouth (Figure 1.87 u). At this important spot we have the source of embryonic development (blastocrene), or "zone of growth," from which the coelomation (and also the gastrulation) originally proceeds.