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.
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_).
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 (Figs. 94 and 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 cœlom-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 cœlom-pouches lies the chorda (Fig. 95 _x_), which here again develops from the middle line of the dorsal wall of the primitive gut.