Fig.137. Three diagrammatic transverse sections of the embryonic disk of the higher vertebrate, to show the origin of the tubular organs from the bending germinal layers. Fig. 137—Three diagrammatic transverse sections of the embryonic disk of the higher vertebrate, to show the origin of the tubular organs from the bending germinal layers. In Fig. _A_ the medullary tube (_n_) and the alimentary canal (_a_) are still open grooves. In Fig. _B_ the medullary tube (_n_) and the dorsal wall are closed, but the alimentary canal (_a_) and the ventral wall are open; the prorenal ducts (_u_) are cut off from the horn-plate (_h_) and internally connected with segmental prorenal canals. In Fig. _C_ both the medullary tube and the dorsal wall above and the alimentary canal and ventral wall below are closed. All the open grooves have become closed tubes; the primitive kidneys are directed inwards. The letters have the same meaning in all three figures: _h_ skin-sense layer, _n_ medullary tube, _u_ prorenal ducts, _x_ axial rod, _s_ primitive-vertebra, _r_ dorsal wall, _b_ ventral wall, _c_ body-cavity or cœloma, _f_ gut-fibre layer, _t_ primitive artery (aorta), _v_ primitive vein (subintestinal vein), _d_ gut-fibre layer, _a_ alimentary canal.
As the two outer layers of the germinative area thus rise in a fold about the embryo, and join above it, they come at last to form a spacious sac-like membrane about it. This envelope takes the name of the germinative membrane, or water-membrane, or _amnion_ (Fig. 142 _am_). The embryo floats in a watery fluid, which fills the space between the embryo and the amnion, and is called the amniotic fluid (Figs. 141, 142 _ah_). We will deal with this remarkable formation and with the allantois later on (Chapter XV). In front of the allantois the yelk-sac or umbilical vesicle (_ds_), the remainder of the original embryonic vesicle, starts from the open belly of the embryo (Fig. 138 _kh_). In more advanced embryos, in which the gastric wall and the ventral wall are nearly closed, it hangs out of the navel-opening in the shape of a small vesicle with a stalk (Figs. 141, 142 _ds_). The more the embryo grows, the smaller becomes the vitelline (yelk) sac. At first the embryo looks like a small appendage of the large embryonic vesicle. Afterwards it is the yelk-sac, or the remainder of the embryonic vesicle, that seems a small pouch-like appendage of the embryo (Fig. 142 _ds_). It ceases to have any significance in the end. The very wide opening, through which the gastric cavity at first communicates with the umbilical vesicle, becomes narrower and narrower, and at last disappears altogether. The _navel,_ the small pit-like depression that we find in the developed man in the middle of the abdominal wall, is the spot at which the remainder of the embryonic vesicle (the umbilical vesicle) originally entered into the ventral cavity, and joined on to the growing gut.