XI.] EMBRYONIC MEMBRANES. 345 



FrvK Diagrammatic Figdkes iLLUsiRATiirG the Formation 

 OF THE Foetal Membranes of a Mammal. (From Kolli- 

 ker.) 



In 1, 2, 3, 4 the embryo is represented in longitudinal section. 



1. Owim with zona pellucida, blastodermic vesicle, and 

 embryonic area. 



2. Ovum with commencing formation of umbilical vesicle 

 and amnion. 



3. Ovum with amnion about to close, and commencing 



allantois. 



4. Ovum with villous subzonal membrane, larger allantois, 

 and mouth and anus. 



5. OvTim in which the mesoblast of the allantois has ex- 

 tended round the inner surface of the subzonal membrane and 

 united with it to form the chorion. The cavity of the allantois 

 is aborted. This fig. is a diagram of an early h\uiian ovum. 



d. zona radiata ; d and sz. processes of zona ; sh. subzonal mem- 

 brane, outer fold of amnion, false amnion ; ch. chorion ; ch. z. 

 chorionic viUi ; am. amnion ; ks. head-fold of amnion ; ss. tail- 

 fold of amnion; a. epiblast of embryo ; a. epiblast of non-em- 

 bryonic part of the blastodermic vesicle ; m. embryonic meso- 

 blast ; m!. non-embryonic mesoblast ; df. area vasculosa ; st. 

 sinlis terminalis; dd. embryonic hypoblast ; i. non-embryo- 

 nic hypoblast ; hh. cavity of blastodermic vesicle, the greater 

 part of which becomes the cavity of umbilical vesicle ds.; 

 dg. stalk of umbilical vesicle ; al. allantois ; e. embryo ; r. 

 space between chorion and amnion containing albuminous 

 . fluid ; vl. ventral body wall ; hh. pericardial cavity. 



