We find in the construction of the ventral wall precisely the same processes as in the formation of the dorsal wall (Fig. 137 _B,_ Fig. 144 _hp,_ Fig. 146 _bh_). It is formed on the flat embryonic shield of the amniotes from the upper plates of the parietal zone. The right and left parietal plates bend downwards towards each other, and grow round the gut in the same way as the gut itself closes. The outer part of the lateral plates forms the ventral wall or the lower wall of the body, the two lateral plates bending considerably on the inner side of the amniotic fold, and growing towards each other from right and left. While the alimentary canal is closing, the body-wall also closes on all sides. Hence the ventral wall, which encloses the whole ventral cavity below, consists of two parts, two lateral plates that bend towards each other. These approach each other all along, and at last meet at the navel. We ought, therefore, really to distinguish two navels, an inner and an outer one. The internal or intestinal navel is the definitive point of the closing of the gut wall, which puts an end to the open communication between the ventral cavity and the cavity of the yelk-sac (Fig. 105). The external navel in the skin is the definitive point of the closing of the ventral wall; this is visible in the developed body as a small depression.
Figs. 145 and 146. Transverse sections of embryos (of chicks). Figs. 145–146—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.