78 



HANDBOOK OF PHYSIOLOGY. 



a mere pedicle which passes out of the body of the embryo at the point 

 of the future umbilicus. 



The downwardly folded portions of blastoderm are termed the vis- 

 ceral plates. 



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 up- 

 ward growth of the dorsal laminae, the difference being that, in the vis- 

 ceral 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, inclosing 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 communi- 

 cates freely with the cavity of the yolk-sac through the canal termed 

 vitelline or omphalo-mesenteric duct. 



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

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



Fig. 471. 



a* 



Fig. 472. 



Figs. 471, 472 and 473. Diagrams showing three successive stages of development. Trans- 

 verse vertical sections. The yolk-sac, ys, is seen progressively diminishing in size. In the 

 embryo itself the medullary canal and notochord are seen in section, a', in middle figure, the 

 alimentary canal, becoming pinched off, as it were, from the yolk-sac ; a' in right-hand figure, 

 alimentary canal completely closed ; a, in last two figures, amnion ; ac, cavity of amnion filled 

 with amniotic fluid ; p, space between amnion and chorion continuous with the pleuro-perito- 

 neal cavity inside the body; vt. vitelline membrane; ys, yolk-sac, or umbilical vesicle. (Foster 

 and Balfour.) 



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

 remaining for some time as the umbilical vesicle (fig. 473, ys.). The 

 hypoblast 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. 



At the posterior end of the embryo chick, when the amniotic fold is 

 commencing to be formed, and the hind fold of the splanchnopleure has 

 commenced, there remains for a time a communication between the 

 neural canal and the hind gut, which is called the neurenteric canal. 



