660 REPRODUCTIVE ORGANS. 



The true amnion has epiblast for its inner, and mesoblast for its 

 outer, layer, and the space between it and the embryo is the 

 araniotic cavity, in which the liquor amnii accumulates. 



Yolk-sac. The yolk-sac is a very, important structure in the 

 fowl and in birds generally, as it is upon the yolk that the nutri- 

 tion of the embryo depends ; but in mammals it is of little impor- 

 tance, as the nutritive material in the vitellus is insignificant in 

 amount. 



Allantois. The allantois is a projection of the splanch- 

 nopleure into the pleuroperitoneal cavity. It subsequently com- 

 municates with the posterior portion of the intestinal canal, and 

 its lining is hypoblast. This structure projects more and more 

 into the pleuroperitoneal cavity, following up the folds that have 

 been described as forming the true and the false amnion. The 

 allantois at last comes in contact with the chorion, which, it will 

 be remembered, \vas formed by the fusion of the false amnion 

 with the vitelline membrane, and into the villi of that structure it 

 sends processes. It is especially developed in that part corre- 

 sponding to the attachment of the ovum to the uterine wall. The 

 allantois has two layers, a mesoblastic and a hypoblastic. In the 

 former are blood-vessels which come from the vascular system of 

 the embryo, the connecting vessels becoming the umbilical arteries. 

 At a later stage of development the character of the allantois dis- 

 appears, except in that portion which is to be included within the 

 body of the fetus, and which becomes the urinary bladder, and in 

 that portion between the bladder and the umbilicus, which becomes 

 the urachus. 



Chorion. This membrane, as already stated, is formed by 

 the union of the vitelline membrane and the false amnion. When 

 first formed, it is smooth, but becomes shaggy by the growth from 

 it of processes called villi. These villi are at first scattered over 

 the whole exterior of the ovum, but later they are found only at 

 the point of attachment of the ovum to the uterus, where 'the 

 placenta is to be formed. In these villi are blood-vessels from the 

 fetal vascular system. 



Placenta. When the impregnated ovum reaches the cavity 

 of the uterus the mucous membrane of that organ is prepared to 

 receive it, and it finds a lodgment there. Under the stimulus of 

 impregnation the whole mucous membrane becomes thickened, and 

 at the termination of uterogestation the entire mucous membrane 

 of the body is cast off; it is called the decidua vera. Especially 

 marked is this thickening at the point of attachment of the ovum, 

 and to this part the name decidua serotina is applied (Fig. 443). 

 As a result of this stimulus the mucous membrane increases around 

 the ovum, finally completely enclosing it. This new formation is 

 the decidua reflexa. 



The villi of the chorion find their way into the depressions of 



