574 



ZOOLOGY 



SECT. 



.extend into them later, so that, either from the first, or as a result 

 of outgrowth which takes place subsequently, the chorion contains 

 mesoderm as well as ectoderm. The ectodermal cells trophoblast 

 cells of the chorion may enter into close relationship with the 

 mucous membrane of the wall of the uterus, and send out processes 

 or primary villi (Fig. 1231, EK) by means of which the ovum 

 becomes intimately attached, and by means of which perhaps 

 nourishment is absorbed. 



TA 



AX 



SI 



El 



OL 



AN 



H 



EK 



IG. 1231. A Rabbit 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 ccelome ; 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 ; El. auditory vesicle ; EK, ectodermal villi ; OF. fore-gut ; GH. 

 hind-gut; GT. mid-gut; H. endoderm ; 0, extra-embryonic ccelome; OL. lens of eye; 

 H. heart ; SI. sinus-terminalis ; TA. allantoic cavity ; YS. yolk-sac. (From Marshall, in 

 part after Van Beneden and Julin.) 



In certain Mammals the history of the amnion is very different 

 from that above described. In the Hedgehog (Fig. 1232), for 

 example, a cavity appears in the ectoderm of the embryonic area ; 

 this is destined to give rise to the cavity of the amnion. The 

 ectoderm, which forms its roof, is entirely trophoblastic or chorionic ; 

 that which forms its floor is partly destined to become amniotic 

 ectoderm, partly embryonal ectoderm. After the mesoderm has 

 begun to become differentiated, the margins of the amniotic part 



