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). 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).