The accumulation of food-yelk in the ventral wall of the primitive gut (Figs. 85, 86) is the simple cause that converts the sac-shaped cœlom-pouches of the acrania into the leaf-shaped cœlom-streaks of the craniotes. To convince ourselves of this we need only compare, with Hertwig, the palingenetic cœlomula of the amphioxus (Figs. 80, 81) with the corresponding cenogenetic form of the amphibia (Figs. 89–90), and construct the simple diagram that connects the two (Figs. 87, 88). If we imagine the ventral half of the primitive gut-wall in the amphioxus embryo (Figs. 79–84) distended with food-yelk, the vesicular cœlom-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 (Figs. 86, 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 (Fig. 87 u). At this important spot we have the source of embryonic development (blastocrene), or “zone of growth,” from which the cœlomation (and also the gastrulation) originally proceeds.
Fig. 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 Kölliker.) 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 cœlom-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 cœlomula-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 (Figs. 87, 88). In sections both through the primitive mouth itself (Fig. 89) and in front of it (Fig. 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 (Fig. 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 cœlomula-embryos of the amphibia (Fig. 91) as in the amphioxus (Figs. 79–82). It arises from the entodermic cell-streak, which forms the middle dorsal-line of the primitive gut, and occupies the space between the flat cœlom-pouches (Fig. 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 (Fig. 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) (Fig. 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 (Fig. 91 A, ch); these now alone form the clothing of the visceral cavity (dh) (enteroderm, Fig. 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 (Figs. 85–91) to see that, as a fact, the cenogenetic cœlomation of the amphibia can be deduced directly from the palingenetic form of the acrania (Figs. 79–84).
Fig. 93—Transverse section of the vertebrate-embryo of a bird (from a hen’s egg on the second day of incubation). (From Kölliker.) h horn-plate, mr medullary tube, ch chorda, uw primitive segments, uwh primitive-segment cavity (median relic of the cœlom), sp lateral cœlom-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 cœlomation 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 cœlomation 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 Kölliker 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 cœlom-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 (Fig. 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 (Fig. 93 mr).