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). 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.