592 



ZOOLOGY 



SECT. 



around the embryo, and grow upwards and inwards, eventually 

 meeting in the middle over the body of the embryo, and 

 uniting in such a way as to form two layers. Of the two layers 

 thus formed the outer, consisting of trophoblastic ectoderm and 

 somatic mesoderm, simply constitutes a part of the extra- 

 embryonic somatopleure which forms a complete investment for 



OF 



TA 



AX 



OL 



YS 



AN 



EK 



FIG. 1219. A Babbit embryo and blastodermic vesicle at the end of the tenth day. The 

 embryo is represented in surface view from the right side, the course of the alimentary 

 canal being indicated by the broad dotted line ; the blastodermic vesicle is shown in median 

 longitudinal section. The greater part of the tail has been removed. AN', pro-amnion ; 

 AX. cavity of amnion ; C. extra-embryonic portion of coelome ; E. ectoderm ; E'. thickened 

 ectoderm by which the vesicle is attached to the uterus and from which the foetal part 

 of the placental is derived ; EGT. ectodermal villi ; El. auditory vesicle ; EK, ectodermal 

 villi ; GF. fore-gut ; GH. hind-gut ; GT. mid-gut ; H. eiidoderm ; OL. lens of eye ; R. heart ; 

 SI. sinus] terminalis ; TA. allantoic cavity ; YS. yolk-sac. (From Marshall, in part after Van 

 Beneden and Julin.) 



the entire sphere, and is known as the chorion (Fig. 1218, 2 and 3). 

 In the account of the development of the Bird it has been 

 referred to as the false amnion or serous membrane. The inner 

 layer or true amnion, as in the Bird, forms the wall of a cavity 

 the amniotic cavity (4 and 5, ah) which becomes tensely filled 

 with fluid (the liquor amnii) over the body of the embryo ; this 

 serves the purpose of protecting the delicate embryo from the 



