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.)
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).
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.
