558 



ZOOLOGY 



SECT 



ch 



FIG. 1149. Five diagrammatic sections illustrating the formation of the foetal membranes 

 of a Mammal. 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 ; 4, ovum 

 with villous subzonal membrane, larger allantois, and mouth and anus ; 5, ovum in which 

 the mesoderm of the allantois has extended round the inner surface of the subzonal mem- 

 brane and united with it to form the chorion ; the cavity of the allantois is aborted ; 

 a, ectoderm of embryo ; a', ectoderm of non-embryonic part of the blastodermic vesicle ; 

 ah. amniotic cavity ; al. allantois ; am. amnion ; ch. chorion ; chz, chorionic villi ; d, zona 

 radiata ; d'. processes of zona ; dd, embryonic endoderm ; df. area vasculosa ; dg, stalk of 

 umbilical vesicle ; ds, cavity of umbilical vesicle ; e. embryo ; hh, pericardial cavity ; i, non- 

 embryonic endoderm ; Kh, cavity of blastodermic vesicle ; Ks. head-fold of amnion ; m. em- 

 bryonic mesoderm ; m'. non-embryonic mesoderm ; r, space between chorion and amnion ; 

 sh, subzonal membrane ; ss, tail-fold of amnion ; st, sinus terminalis ; vl. ventral body wall 

 (From Balfour, after Kolliker.) 



