DEVELOPMENT. 



295 



FIG. 679. Diagramatic- figures illustrating the formation of the foetal membranes of a 

 Mammal (after Kolliker). a, Ovum with first rudiments of embryo; b, Ovum with 

 yolk-sac and developing amnion ; c, Ovum with amnion closing and developing allantois; 

 d, Ovum with villous serous envelope, embryo with mouth and anus ; e, Ovum in which 

 the vascular layer of the allantois is applied to the serous envelope and has grown into 

 the villi of the latter, yolk-sac reduced, the amniotic cavity is increasing; D, Zona 

 radiata; D', Villi of zona ; Sh, subzonal membrane (serous envelope) ; Sz, Villi of sub- 

 zonal membrane ; Ch, Chorion (vascular layer of Allantois) ; Chz, Chorionic villi (con- 

 sisting of chorion and subzonal membrane) ; Am, Amnion ; Ah, Amniotic cavity ; 

 E, Embryo ; A, Embryonic thickening of the external layer ; M, of the middle layer ; J, 

 of the inner layer ; Ds, cavity of the embryonic (blastodermic) vesicle, later of the yolk- 

 sac (umbilical vesicle) ; Dh, Intestinal cavity ; Dg, Umbilical stalk ; Al, Allantois. 



