The origin of the navel coincides with the complete closing of the external ventral wall. In the amniotes the ventral wall originates in the same way as the dorsal wall. Both are formed substantially from the skin-fibre layer, and externally covered with the horn-plate, the border section of the skin-sense layer. Both come into existence by the conversion of the four flat germinal layers of the embryonic shield into a double tube by folding from opposite directions; above, at the back, we have the vertebral canal which encloses the medullary tube, and below, at the belly, the wall of the body-cavity which contains the alimentary canal (Fig. 137). Figs. 138 to 142. Five diagrammatic longitudinal sections of the maturing mammal embryo and its envelopes. Figs. 138–142—Five diagrammatic longitudinal sections of the maturing mammal embryo and its envelopes. In Figs. 138–141 the longitudinal section passes through the sagittal or middle plane of the body, dividing the right and left halves; in Fig. 142 the embryo is seen from the left side. In Fig. 138 the tufted it prochorion (_dd′_) encloses the germinal vesicle, the wall of which consists of the two primary layers. Between the outer (_a_) and inner (_i_) layer the middle layer (_m_) has been developed in the region of the germinative area. In Fig. 139 the embryo (_e_) begins to separate from the embryonic vesicle (_ds_), while the wall of the amnion-fold rises about it (in front as head-sheath, _ks,_ behind as tail-sheath, _ss_). In Fig. 140 the edges of the amniotic fold (_am_) rise together over the back of the embryo, and form the amniotic cavity (_ah_); as the embryo separates more completely from the embryonic vesicle (_ds_) the alimentary canal (_dd_) is formed, from the hinder end of which the allantois grows (_al_). In Fig. 141 the allantois is larger; the yelk-sac (_ds_) smaller. In Fig. 142 the embryo shows the gill-clefts and the outline of the two legs; the chorion has formed branching villi (tufts.) In all four figures _e_=embryo, _a_ outer germinal layer, _m_ middle germinal layer, _i_ inner germinal layer, _am_ amnion (_ks_ head-sheath, _ss_ tail-sheath), _ah_ amniotic cavity, _as_ amniotic sheath of the umbilical cord, _kh_ embryonic vesicle, _ds_ yelk-sac (umbilical vesicle), _dg_ vitelline duct, _df_ gut-fibre layer, _dd_ gut-gland layer, _al_ allantois, _vl=hh_ place of heart, _d_ vitelline membrane (ovolemma or prochorion), _d′_ tufts or villi of same, _sh_ serous membrane (serolemma), _sz_ tufts of same, _ch_ chorion, _chz_ tufts or villi, _st_ terminal vein, _r_ pericœlom or serocœlom (the space, filled with fluid, between the amnion and chorion). (From _Kölliker._)