350 EMBRYONIC MEMBRANES AND YOLK-SAC. [CHAP. 



meantime made their appearance in the walls of the 

 uterus 1 . The villi of the chorion are covered by an 

 epithelium derived from the subzonal membrane, and 

 are provided with a connective-tissue core containing 

 an artery and vein and a capillary plexus connecting 

 them. In most cases they assume a more or less ar- 

 borescent form, and have a distribution on the surface 

 of the chorion varying characteristically in different 

 species. The walls of the crypts into which the villi 

 are fitted also become highly vascular, and a nutritive 

 fluid passes from the maternal vessels of the placenta 

 to the foetal vessels by a process of diffusion; while 

 there is probably also a secretion by the epithelial 

 lining of the walls of the crypts, which becomes ab- 

 sorbed by the vessels of the fcetal villi. The above 

 maternal and foetal structures constitute together the 

 organ known as the placenta. The maternal portion 

 consists essentially of the vascular crypts in the 

 uterine walls, and the foetal portion of more or less 

 arborescent villi of the true chorion fitting into these 

 crypts. 



While the placenta is being developed the folding 

 off of the embryo from the yolk-sac becomes more 

 complete; and the yolk-sac remains connected with the 

 ileal region of the intestine by a narrow stalk, the vi- 

 telline duct (Fig. 114, 4 and 5 and Fig. 115), consisting 

 of the same tissues as the yolk-sac, viz. hypoblast and 

 splanchnic mesoblast. While the true splanchnic stalk 



1 These crypts have no connection with the openings of glands in 

 the walls of the uterus. They are believed by Ercolani to be formed 

 to a large extent by a regeneration of the lining tissue of the uterine 

 walls. 



