780 HANDBOOK OF PHYSIOLOGY. 



birds, the contents of the yolk-sac afford nourishment until the end of 

 incubation, and the omphalo-mesenteric vessels are developed to a corre- 

 sponding degree; but in mammalia the office of the umbilical vesicle 

 ceases at a very early period, as the quantity of the yolk 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 gradu- 

 ally drawn into the abdomen through the umbilical opening, which then 



Fig. 474. Fig. 475. 



Fig, 474. Diagram showing vascular area in the chick, a, area pellucida; 6, area vasculosa; 

 c, area vitellina. 



Fig. 475. Human embryo of fifth week with umbilical vesicle; about natural size. GDalton.) 

 The human umbilical vesicle never exceeds the size of a small pea. 



closes over it: but in mammalia it always remains on the outside; and 

 as it is emptied it contracts (fig. 473), shrivels up, and together with 

 the part of its duct external to the abdomen, is detached 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 developed, they ramify largely over 

 the walls of the umbilical vesicle, and are actively concerned in absorb- 

 ing its contents and conveying them away for the nutrition of the 

 embryo. 



At an early stage of development of the fcetus, and some time before 

 the completion of the changes which have been just described, two im- 

 portant structures, called respectively the amnion and the attantois, begin 

 to be formed. 



Amnion. The amnion is produced as follows: Beyond the head- 

 and tail-folds before described (p. 776), the somatopleure coated by epi- 

 blast, is raised into folds, which grow up, arching over the embryo, not 

 only anteriorly and posteriorly but also laterally, and all converging 

 toward one point over its dorsal surface (fig. 470). The growing up of 

 these folds from all sides and their convergence toward one point very 

 closely resembles the folding inward of the visceral plates already de- 

 scribed, and hence, by some, the point at which the amniotic folds 

 meet over the back has been termed the amniotic umbilicus. 



The folds not only come into contact but coalesce. The inner of 



