82O DEVELOPMENT. [CH. LVIII. 



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 

 mammalia it always remains on the outside ; and as it is 

 emptied it contracts (fig. 635), 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. 



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

 before the completion of the changes which have been just 

 described, two important structures, called respectively the 

 amnion and the allantois, begin, to be formed. 



Amnion. The amnion is produced as follows : Beyond the 

 head- and tail-folds before described (p. 812), the somatopleur 

 coated by epiblast, is raised into folds, which grow up, arching 

 over the embryo, not only anteriorly and posteriorly but also 

 laterally, and all converging towards one point over its dorsal 

 surface (fig. 637). 



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

 the two layers forms the true amnion ; it is composed externally of 

 mesoblast and is lined by epiblast ; the outer layer is termed the 

 false amnion ; it is composed externally of epiblast and is lined 

 by mesoblast. It coalesces with the inner surface of the remains 

 of the original vitelline membrane or zona pellucida. 



The cavity between the true amnion and the external surface 

 of the embryo becomes a closed space, termed the amniotic cavity 

 (ac, fig. 635). 



At first, the amnion closely invests the embryo, but it becomes 

 gradually distended with fluid (liquor amnii), which, as pregnancy 

 advances, reaches a considerable quantity. 



This fluid consists of water containing small quantities of 

 albumin, urea, and salts. Its chief function during gestation 

 appears to be the mechanical one of affording equal support to the 

 embryo on all sides, and of protecting it as far as possible from 

 the effects of blows and other injuries to the abdomen of the 

 mother. It is an exudation from both foetal and maternal blood; 

 the urea in it comes from the foetal urine, which is passed into it 

 iu the later months of pregnancy. 



On referring to figs. 633, 634 and 635, it will be obvious that 



