’ 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-bhag and the allantois takes 
place in Mammals (fig. 324) almost exactly on the same plan 
Fig. 324.—Empryro oF Mote: 
A, entire; B, with the abdomen laid open :—a, chorion ; 5, footstalk of allantois and 
umbilical vessels; c, yolk-bag; d, vitelline duct; e, upper portion of intestinal 
canal; f, lower portion; g, eye; &, indication of branchial arches; i, auditory 
yep: (F Pcp oe ee posterior extremity; m,n, Wolffian bodies or rudi- 
un YS; 0, entary lung; p, trachea; g, ventricle of heart; r, 
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 
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 
