8o8 



EMBRYOLOGY 



embryo, the gelatinous mass already described being situated between 

 it and the internal amniotic layer before this layer becomes enlarged. 

 While the formation 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 formation, 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 different 

 stages of the development of the allantois are shown in the diagram- 

 matic Fig. 224(3, 4, 5)- 



It is the vascularity of the allan- 

 tois that causes the rapid develop- 

 ment 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 arteries persist 

 and form the two arteries of the um- 

 bilical cord, coming from the internal 

 iliac arteries of the foetus ; 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 presents 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 embryo, the villi are developed more rapidly than over the rest 

 of the surface. Indeed, as the ovum 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 of the allantois 

 over which the villi persist and increase in length and in the number of 

 their branches is destined to form connections with the mucous mem- 

 brane of the uterus and constitutes the foetal portion of the placenta. 



Fig. 226. Human embryo of the fourth 

 to the fifth -week, X 2 prepared by Dr. C. 

 R. Cor son, of Savannah, Ga. (Gage). 



This ovum shows the shaggy chorion, 

 which has been split open and the edges of 

 the opening drawn aside to show the em- 

 bryo. The embryo is curved as usual, and 

 the head rests on the prominent heart and is 

 almost in contact with the pear-shaped 

 umbilical vesicle. The arm-bud and the 

 branchial slits show clearly. The somites 

 (protovertebrae) may be seen on close in- 

 spection. 



