262 HAND-BOOK OF PHYSIOLOGY. 



vessels (omphalo-mesenteric) which ramify in the walls of the yelk-sac, 

 forming what in birds is termed the area vasculosa. In Birds, the con- 

 tents of the yelk-sac afford nourishment until the end of incubation, and 

 the omphalo-mesenteric vessels are developed to a corresponding degree; 

 but in Mammalia the office of the umbilical vesicle ceases at a very early 

 period, the quantity of the 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 abdomen 

 through the umbilical opening, which then closes over it: but in Mam- 



Fio. 418. FIG. 419. 



FIG. 418. Diagram showing vascular area in the chick, a, area pellucida; &, area vasculosa; c y 

 area vitellina. 



FIG. 419. Human embryo of fifth week with umbilical vesicle; about natural size (Dalton). The 

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



malia it always remains on the outside; and as it is emptied it contracts 

 (Fig. 419), 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 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 

 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. 259, Vol. II.), the somatopleure coated by 

 epiblast, is raised into folds, which grow up, arching over the embryo, 

 not only anteriorly and posteriorly but also laterally, and all converg- 

 ing toward one point over its dorsal surface (Fig. 417). 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 

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

 meet over the back has been termed the "amniotic umbilicus." 



