604 AMNION AND ALLANTOIS. 



tinues, it passes this point, and embraces more and more of the in- 

 ferior, as well as of the superior hemisphere, the vessels converging 

 toward its under surface, until at last nearly the whole of the 

 vitellus is covered with a network of inosculating capillaries. 



The function of the vessels of the area vasculosa is to absorb 

 nourishment from the cavity of the vitelline sac. As the albumen 

 liquefies during the process of incubation, it passes by endosmosis, 

 more and more abundantly, into the vitelline cavity ; the whole 

 vitellus growing constantly larger and more fluid in consistency. 

 The blood of the foetus, then circulating in the vessels of the area 

 vasculosa, absorbs freely the oleagino-albuminous matters of the 

 vitellus, and, carrying them back to the foetus by the returning 

 veins, supplies the newly-formed tissues and organs with abun- 

 dance of appropriate nourishment. 



During this period the amnion and the allantois have been also 

 in process of formation. At first the body of the foetus lies upon 

 its abdomen, as in the cases previously described ; but, as it increases 

 in size, it alters its position so as to lie more upon its side. The 

 allantois then, emerging from the posterior portion of the abdominal 

 cavity, turns directly upward over the body of the foetus, and comes 

 immediately in contact with the shell membrane. (Fig. 212.) It 



Fig. 212. 



Eaa OF FOWL at a more advanced period of development. The body of the foetus is enveloped 

 by the amnion, and has the umbilical vesicle hanging from its under surface; while the vascular 

 allantois is *een turning upward and spreading out over the internal surface of the shell-membrane. 



then spreads out rapidly, extending toward the extremities and 

 down the sides of the egg, enveloping more and more completely 



