XI.] EMBRYONIC MEMBRANES. 345 



Five Diagrajimatic Figures illustrating the Formation 

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

 ker.) 



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



1. Ovum 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. Ovum 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 human 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 villi ; 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. 

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

 nic hypoblast ; Ich. 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. 



