The accumulation 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.
(FIGURE 1.92. Transverse section of the chordula-embryo of a bird (from a hen's egg at the close of the first day of incubation). (From Kolliker,) h horn-plate (ectoderm), m medullary plate, Rf dorsal folds of same, Pv medullary furrow, ch chorda, uwp median (inner) part of the middle layer (median wall of the coelom-pouches), sp lateral (outer) part of same, or lateral plates, uwh structure of the body-cavity, dd gut-gland-layer.)
Hertwig even succeeded in showing, in the coelomula-embryo of the water salamander (Triton), between the first structures of the two middle layers, the relic of the body-cavity, which is represented in the diagrammatic transitional form (Figures 1.87 and 1.88). In sections both through the primitive mouth itself (Figure 1.89) and in front of it (Figure 1.90) the two middle layers (pb and vb) diverge from each other, and disclose the two body-cavities as narrow clefts. At the primitive-mouth itself (Figure 1.90 u) we can penetrate into them from without. It is only here at the border of the primitive mouth that we can show the direct transition of the two middle layers into the two limiting layers or primary germinal layers.
The structure of the chorda also shows the same features in these coelomula-embryos of the amphibia (Figure 1.91) as in the amphioxus (Figures 1.79 to 1.82). It arises from the entodermic cell-streak, which forms the middle dorsal-line of the primitive gut, and occupies the space between the flat coelom-pouches (Figure 1.91 A). While the nervous centre is formed here in the middle line of the back and separated from the ectoderm as "medullary tube," there takes place at the same time, directly underneath, the severance of the chorda from the entoderm (Figure 1.91 A, B, C). Under the chorda is formed (out of the ventral entodermic half of the gastrula) the permanent gut or visceral cavity (enteron) (Figure 1.91 B, dh). This is done by the coalescence, under the chorda in the median line, of the two dorsal side-borders of the gut-gland-layer (ik), which were previously separated by the chorda-plate (Figure 1.91 A, ch); these now alone form the clothing of the visceral cavity (dh) (enteroderm, Figure 1.91 C). All these important modifications take place at first in the fore or head-part of the embryo, and spread backwards from there; here at the hinder end, the region of the primitive mouth, the important border of the mouth (or properistoma) remains for a long time the source of development or the zone of fresh construction, in the further building-up of the organism. One has only to compare carefully the illustrations given (Figures 1.85 to 1.91) to see that, as a fact, the cenogenetic coelomation of the amphibia can be deduced directly from the palingenetic form of the acrania (Figures 1.79 to 1.84).
(FIGURE 1.93. Transverse section of the vertebrate-embryo of a bird (from a hen's egg on the second day of incubation). (From Kolliker.) h horn-plate, mr medullary tube, ch chorda, uw primitive segments, uwh primitive-segment cavity (median relic of the coelom), sp lateral coelom-cleft, hpl skin-fibre-layer, df gut-fibre-layer, ung primitive-kidney passage, ao primitive aorta, dd gut-gland-layer.)
The same principle holds good for the amniotes, the reptiles, birds, and mammals, although in this case the processes of coelomation are more modified and more difficult to identify on account of the colossal accumulation of food-yelk and the corresponding notable flattening of the germinal disk. However, as the whole group of the amniotes has been developed at a comparatively late date from the class of the amphibia, their coelomation must also be directly traceable to that of the latter. This is really possible as a matter of fact; even the older illustrations showed an essential identity of features. Thus forty years ago Kolliker gave, in the first edition of his Human Embryology (1861), some sections of the chicken-embryo, the features of which could at once be reduced to those already described and explained in the sense of Hertwig's coelom-theory. A section through the embryo in the hatched hen's egg towards the close of the first day of incubation shows in the middle of the dorsal surface a broad ectodermic medullary groove (Figure 1.92 Rf), and underneath the middle of the chorda (ch) and at each side of it a couple of broad mesodermic layers (sp). These enclose a narrow space or cleft (uwh), which is nothing else than the structure of the body-cavity. The two layers that enclose it—the upper parietal layer (hpl) and the lower visceral layer (df)—are pressed together from without, but clearly distinguishable. This is even clearer a little later, when the medullary furrow is closed into the nerve-tube (Figure 1.93 mr).
Special importance attaches to the fact that here again the four secondary germinal layers are already sharply distinct, and easily separated from each other. There is only one very restricted area in which they are connected, and actually pass into each other; this is the region of the primitive mouth, which is contracted in the amniotes into a dorsal longitudinal cleft, the primitive groove. Its two lateral lip-borders form the primitive streak, which has long been recognised as the most important embryonic source and starting-point of further processes. Sections through this primitive streak (Figures 1.94 and 1.95) show that the two primary germinal layers grow at an early stage (in the discoid gastrula of the chick, a few hours after incubation) into the primitive streak (x), and that the two middle layers extend outward from this thickened axial plate (y) to the right and left between the former. The plates of the coelom-layers, the parietal skin-fibre-layer (m) and the visceral gut-fibre-layer (f), are seen to be still pressed close together, and only diverge later to form the body-cavity. Between the inner borders of the two flat coelom-pouches lies the chorda (Figure 1.95 x), which here again develops from the middle line of the dorsal wall of the primitive gut.
(FIGURES 1.94 AND 1.95. Transverse section of the primitive-streak (primitive mouth) of the chick. Figure 1.94 a few hours after the commencement of incubation, Figure 1.95 a little later. (From Waldeyer.) h horn-plate, n nerve-plate, m skin-fibre-layer, f gut-fibre-layer, d gut-gland-layer, y primitive streak or axial plate, in which all four germinal layers meet, x structure of the chorda, u region of the later primitive kidneys.)
Coelomation takes place in the vertebrates in just the same way as in the birds and reptiles. This was to be expected, as the characteristic gastrulation of the mammal has descended from that of the reptiles. In both cases a discoid gastrula with primitive streak arises from the segmented ovum, a two-layered germinal disk with long and small hinder primitive mouth. Here again the two primary germinal layers are only directly connected (Figure 1.96 pr) along the primitive streak (at the folding-point of the blastula), and from this spot (the border of the primitive mouth) the middle germinal layers (mk) grow out to right and left between the preceding. In the fine illustration of the coelomula of the rabbit which Van Beneden has given us (Figure 1.96) one can clearly see that each of the four secondary germinal layers consists of a single stratum of cells.
Finally, we must point out, as a fact of the utmost importance for our anthropogeny and of great general interest, that the four-layered coelomula of man has just the same construction as that of the rabbit (Figure 1.96). A vertical section that Count Spee made through the primitive mouth or streak of a very young human germinal disk (Figure 1.97) clearly shows that here again the four secondary germ-layers are inseparably connected only at the primitive streak, and that here also the two flattened coelom-pouches (mk) extend outwards to right and left from the primitive mouth between the outer and inner germinal layers. In this case, too, the middle germinal layer consists from the first of two separate strata of cells, the parietal (mp) and visceral (mv) mesoblasts.