Figs. 143 and 144. Transverse sections of embryos (of chicks). Figs. 143–144—Transverse sections of embryos (of chicks). Fig. 143 of the second, Fig. 144 of the third, Fig. 145 of the fourth, and Fig. 146 of the fifth day of incubation. Fig. 143–145 from _Kölliker,_ magnified; Fig. 146 from _Remak,_ magnified. _h_ horn-plate, _mr_ medullary tube, _ung_ prorenal duct, _un_ prorenal vesicles, _hp_ skin-fibre layer, _m=mu=mp_ muscle-plate, _uw_ provertebral plate (_wh_ cutaneous rudiment of the body of the vertebra, _wb_ of the arch of the vertebra, _wq_ the rib or transverse continuation), _uwh_ provertebral cavity, _ch_ axial rod or chorda, _sh_ chorda-sheath, _bh_ ventral wall, _g_ hind and _v_ fore root of the spinal nerves, _a=af=am_ amniotic fold, _p_ body-cavity or cœloma, _df_ gut-fibre layer, _ao_ primitive aortas, _sa_ secondary aorta, _vc_ cardinal veins, _d=dd_ gut-gland layer, _dr_ gastric groove. In Fig. 143 the larger part of the right half, in Fig. 144 the larger part of the left half, of the section is omitted. Of the yelk-sac or remainder of the embryonic vesicle only a small piece of the wall is indicated below. We will consider the formation of the dorsal wall first, and that of the ventral wall afterwards (Figs. 143–147). In the middle of the dorsal surface of the embryo there is originally, as we already know, the medullary (_mr_) tube directly underneath the horn-plate (_h_), from the middle part of which it has been developed. Later, however, the provertebral plates (_uw_) grow over from the right and left between these originally connected parts (Figs. 145, 146). The upper and inner edges of the two provertebral plates push between the horn-plate and medullary tube, force them away from each other, and finally join between them in a seam that corresponds to the middle line of the back. The coalescence of these two dorsal plates and the closing in the middle of the dorsal wall take place in the same way as the medullary tube, which is henceforth enclosed by the vertebral tube. Thus is formed the dorsal wall, and the medullary tube takes up a position inside the body. In the same way the provertebral mass grows afterwards round the chorda, and forms the vertebral column. Below this the inner and outer edge of the provertebral plate splits on each side into two horizontal plates, of which the upper pushes between the chorda and medullary tube, and the lower between the chorda and gastric tube. As the plates meet from both sides above and below the chorda, they completely enclose it, and so form the tubular, outer chord-sheath, the sheath from which the vertebral column is formed (_perichorda,_ Fig. 137 _C, s_; Figs. 145 _uwh,_ 146).