DEVELOPMENT AND NUTRITION 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 op Mole : 
a, entire; b, with the abdomen laid open : — a, chorion ; &, footstalk of allantois and 
umbilical vessels; c, yolk-bag; d, vitelline duct; e, upper portion of intestinal 
canal; /, lower portion ; g, eye; h , indication of branchial arches ; i, auditory 
vesicle; Jc, anterior, and l, 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; 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 
