FOEMATION OF THE ALLANTOIS. 807 



and soon after, all vascular connection with the embryon is lost. At first 

 there is a canal of communication with the intestine, called the omphalo- 

 mesenteric canal. This is gradually obliterated, and it closes, between the 

 thirtieth and the thirty-fifth day. The point of communication of the vesi- 

 cle with the intestine is called the intestinal umbilicus ; and early in the 

 process of development, there is here a hernia of a loop of intestine. The 

 umbilical vesicle remains as a tolerably prominent structure as late as the 

 fourth or fifth month, but it may often be discovered at the end of preg- 

 nancy. 



The umbilical vesicle presents three coats ; an external, smooth mem- 

 brane, formed of connective tissue, a middle layer of transparent, polyhedric 

 cells, and an internal layer of spheroidal cells. The membrane, composed of 

 these layers, encloses a pulpy mass, composed of a liquid containing cells and 

 yellowish granulations. 



Formation of the Allantois and the Permanent Chorion. During the 

 early stages of development of the umbilical vesicle, and as it is shut off 

 from the intestine, there appears an elevation at the posterior portion 

 of the intestine, which rapidly increases in extent, until it forms a mem- 

 brane of two layers, which is situated between the internal and the external 

 layers of the amnion. This membrane becomes vascular early in the prog- 

 ress of its development, increases in size quite rapidly, and finally it com- 

 pletely encloses the internal layer of the amnion and the embryon, the 

 gelatinous mass already described being situated between it and the internal 

 amniotic layer before this membrane becomes enlarged. While the forma- 

 tion of the two layers of the allantois is quite distinct in certain of the lower 

 forms of animals, in the human subject and in mammals it is not so easily 

 observed ; still there can be no doubt as to the mechanism of its forma'tion, 

 even in the human ovum. Here, however, the allantois soon becomes a 

 single membrane, the two original layers of which can not be separated from 

 each other. The process of the development of the allantois is shown in the 

 diagrammatic Fig. 295 (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 por- 

 tion of the aorta, and two veins. The two arteries persist and form the two 

 arteries of the umbilical cord, coming from the internal iliac arteries of the 

 fo3tus ; and one vein, the umbilical vein, which returns the blood from the 

 placenta to the foetus, is permanent. These vessels are connected with the 

 permanent, vascular tufts of the chorion. 



The development of the allantois can not be well observed in human ova 

 before the fifteenth or the twenty-fifth day. When the allantois becomes the 

 permanent chorion, it is marked by a large number of hollow, branching 

 villi over its entire surface, which give the ovum a shaggy appearance. As 

 the ovum enlarges, over a certain area surrounding the point of attachment 

 of the pedicle which connects the chorion with the embryon, the villi are 



