DEVELOPMENT AND NUTKITION OF MAMMALIAN EMBRYO. 587 



itself from its shell ; which it does entirely by its own exer- 

 tions. When it thus becomes independent of the allantois, 

 the circulation through the latter diminishes ; and almost the 

 whole sac is separated from the body by the contraction of 

 the connecting foot-stalk, which at last gives way. 



761. The formation of the yolk-bag and the allantois takes 

 place in Mammals (fig. 324) almost exactly on the same plan 



Fig. 324. EMBRYO OF MOLE : 



A, entire ; B, with the abdomen laid open : a, chorion ; b, footstalk of allantois and 

 umbilical vessels; c, yolk-bag; d, vitelline duct; e, upper portion of intestinal 

 canal; /.lower portion; ff, eye; h, indication of branchial arches; f, auditory 

 vesicle; k, anterior, and I, posterior extremity, m, n, Wolffian bodies or rudi- 

 mentary kidneys ; o, rudimentary lung ; p, trachea ; q, ventricle of heart ; r, 

 atrium of heart. 



as in Birds j but on account of the absence of food-yolk, these 

 sacs are comparatively small ; and the function of both is su- 

 perseded, at an early period of the development of the embryo, 

 by a new and remarkable contrivance. The ovum, in passing 

 through the oviduct, has been already stated to receive a new 

 envelope, analogous to that which forms the membrane of the 

 shell in Birds ; this is termed the Chorion. It is then received 

 into the cavity of the Uterus, a receptacle within which it is 

 delayed for a considerable period, and continually supplied 

 with nourishment drawn from the blood of the parent. From 



