GENERATION AND DEVELOPMENT. 



261 



Thus we see that the body-cavity is formed by the downward folding 

 of the visceral plates, just as the neural cavity is produced by the upward 

 growth of the dorsal laminae, the difference being that, in the visceral or 

 ventral laminae, all three layers of the blastoderm are concerned. 



The folding in of the splanchnopleure, lined by hypoblast, pinches 

 off, as it were, a portion of the yelk-sac, enclosing it in the body-cavity. 

 This forms the rudiment of the alimentary canal, which at this period 

 ends blindly toward the head and tail, while in the centre it communicates 

 freely with the cavity of the yelk-sac through the canal termed vitelline 

 or omphalo-mesenteric duct. 



The yelk-sac thus becomes divided into two portions which communi- 

 cate through the vitelline duct, that portion within the body giving rise/ 

 as above stated, to the digestive canal, and that outside the body remain- 

 ing for some time as the umbilical vesicle (Fig. 417, ys). The hypoblasfc 

 forming the epithelium of the intestine is of course continuous with the 

 lining membrane of the umbilical vesicle, while the visceral plate of the 

 mesoblast is continuous with the outer layer of the umbilical vesicle. 



All the above details will be clear on reference to the accompanying 

 diagrams. 



FCETAL MEMBRANES. 



Umbilical Vesicle or Yelk-sac. The splanchnopleure, lined by 

 hypoblast, forms the yelk-sac in Eeptiles, Birds, and Mammals; but in 

 Amphibia and Fishes, since there is neither amnion nor allantois, the 

 wall of the yelk-sac consists of all three layers of the blastoderm, enclosed, 

 of course, by the original vitelline membrane. 



FIG. 417. Diagrams, showing three successive stages of development. Transverse vertical sec- 

 tions. The yelk-sac, ys. is seen progressively diminishing in size. In the embryo itself the medul- 

 lary canal and notochord are seen in section, a', in middle figure, the alimentary canal, becoming 

 pinched off, as it were : from the yelk-sac; a', in right hand figure, alimentary canal completely 

 closed ; a, in last two figures, amnion : ac, cavity of amnion filled with amniotic fluid ; pp, space be- 

 tween amnion and chorion, continuous with the pleuro-peritoneal cavity inside the body; vt, vitel- 

 line membrane; ?/s, yelk-sac, or umbilical vesicle. (Foster and Balfour.) 



The body of the embryo becomes in great measure detached from the 

 yelk-sac or umbilical vesicle, which contains, however, the greater part 

 of the substance of the yelk, and furnishes a source whence nutriment is 

 derived for the embryo. This nutriment is absorbed by the numerous 



