FORMATION OF THE ALLANTOIS AND PERMANENT CHORION. 905 



membrane, the two original layers of which cannot be separated from each other. The 

 process of the development of the allantois is shown in the diagrammatic Figure 291 

 (3, 4, 5). 



It is the vascularity of the allantois which causes the rapid development by which it 

 invades and finally supersedes the external layer of the amnion, becoming the permanent 

 chorion, or external membrane of the ovum. At first there are two arteries extending 

 into this membrane from the lower portion of the aorta, and two veins. The two arte- 

 ries persist and form the two arteries of the umbilical cord, coming from the internal 

 iliac arteries of the foetus ; and the two veins are reduced to one, the umbilical vein, 

 which returns the blood from the placenta to the foetus. These vessels are connected 

 with the permanent vascular tufts of the chorion. 



The development of the allantois cannot be well observed in human ova before the 

 fifteenth or the twenty-fifth day. We have already noted the formation of villosities, 



first upon the vitelline membrane, and 

 next upon the external amniotic mem- 

 brane, and we have seen that both of 

 these membranes are temporary struct- 

 ures. As the vascular allantois en- 

 croaches upon the external amniotic 

 layer, the villosities become vascular; 

 and, when the allantois becomes the per- 

 manent chorion, it is marked by a mul- 

 titude of compound villi over its entire 

 surface, which give the ovum a shaggy 

 appearance. It is difficult to say whether 

 new villi appear upon the allantois, or 

 whether the villi of the amnion are pene- 

 trated by the vessels of the allantois ; 

 but it is certain that the true or perma- 

 nent .chorion presents upon its surface 

 vascular villi. As the ovum enlarges, 

 over a certain area surrounding the point 

 of attachment of the pedicle winch con- 

 nects it with the embryon, the villi are 

 developed more rapidly than over the rest of the surface. Indeed, as the egg becomes 

 larger and larger, the villi of the surface outside of this area become more and more 

 scanty, lose their vascularity, and finally disappear. That portion upon which the villi 

 persist and increase in length and in the number of their branches is destined to form 

 connections with the mucous membrane of the uterus, and it constitutes the foetal portion 

 of the placenta. This change begins at about the end of the second month, and the pla- 

 centa becomes distinctly limited at about the end of the third month. 



It must be remembered that, as the changes progress which result in the formation of 

 the permanent chorion and the limitation of the foetal portion of the placenta, the forma- 

 tion of the umbilical vesicle and the enlargement of the amnion are also going on. The 

 amnion is gradually becoming distended by the increase in the quantity of amniotic 

 fluid. It reaches the internal surface of the chorion at about the end of the fourth month, 

 extends over the umbilical cord to form its external covering, including the cord of the 

 umbilical vesicle, and the umbilical vesicle itself lies in the gelatinous matter between 

 the two membranes. 



At about the beginning of the fifth month, then, the ovum is constituted as follows : 

 The foetus floats freely in the amniotic fluid, attached to the placenta by the umbili- 

 cal cord ; the chorion presents a highly-vascular, thickened, and villous portion, the fcetnl 

 portion of the placenta ; the rest of the chorion is a simple membrane, without villi and 



