194 



FOETAL MEMBRANES. 



Fig. 147. Five diageammatic figukes illdstbating the formation of the 

 FCETAL MEMBRANES OF A Mammal. (From Kolliker.) 

 In 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 extended 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 ; A' . processes of zona ; sh. subzonal membrane ; 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-embryonic part of the blastodermic vesicle ; 



