748 GENERATION AND DEVELOPMENT. 



in great measure detached from the yelk sac or umbilical 

 vesicle, though the cavity of the rudimentary intestine 

 still communicates with it through the vitelline or omphalo- 

 mesenteric duct, and contains part of the yelk substance 

 with which the vesicle was filled. The yelk- sac contains, 

 however, the greater part of the substance of the yelk, 

 and furnishes a source whence nutriment is derived for 

 the embryo. In birds, the contents of the yelk-sac afford 

 nourishment until the end of incubation : but in Mam- 

 malia, the office of the corresponding umbilical vesicle 

 ceases at a very early period, the quantity of yelk is small, 

 and the embryo soon becomes independent of it by the 

 connections it forms with the parent. Moreover, in birds, 

 as the sac is emptied, it is gradually drawn into the abdo- 

 men through the umbilical opening, which then closes over 

 it : but in Mammalia it always remains on the outside ; 

 Fig. 213.* and as it is emptied it contracts (fig. 2 13), 



shrivels up, and together with the part of 

 its duct external to the abdomen, is de- 

 tached and disappears either before, or 

 at the termination of intra- uterine life, 

 the period of its disappearance varying in 

 different orders of Mammalia. 



When blood-vessels begin to be de- 

 veloped, they ramify largely over the 

 walls of the umbilical vesicle, and are actively concerned 

 in absorbing its contents and conveying them away for the 

 nutrition of the embryo. 



The Amnion and Allantois. 



At an early stage of development of the foetus, and some 

 time before the completion of the changes which have been 

 just described, two important structures, called respectively 



* Fig. 213. Human embryo with umbilical vesicle ; about the fifth 

 week (after Dalton). 



