(FIGURES 1.138 TO 1.142. Five diagrammatic longitudinal sections of the maturing mammal embryo and its envelopes. In Figures 1.138 to 1.141 the longitudinal section passes through the sagittal or middle plane of the body, dividing the right and left halves; in Figure 1.142 the embryo is seen from the left side. In Figure 1.138 the tufted it prochorion (dd apostrophe) 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 Figure 1.139 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 Figure 1.140 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 Figure 1.141 the allantois is larger; the yelk-sac (ds) smaller. In Figure 1.142 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 apostrophe tufts or villi of same, sh serous membrane (serolemma), sz tufts of same, ch chorion, chz tufts or villi, st terminal vein, r pericoelom or serocoelom (the space, filled with fluid, between the amnion and chorion). (From Kolliker.)) 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 (Figure 1.137).