752 



GENERATION AND DEVELOPMENT. 



by vessels united in a circulus venosus, or sinus terminates, 

 but it soon extends over the whole surface of the germinal 

 membrane. 



At about the same time, the rudimentary heart is formed 

 in the same layer of the germinal membrane. As shown 

 by Schwann, the blood-vessels are developed originally 

 from nucleated cells. These cells send out processes ; the 

 processes from different cells unite ; and in this way rami- 

 fications and a network are produced vessels extending 

 from this network in the area vasculosa into the area 

 pellucida, and joining the rudimentary heart (see p. 765). 



The Chorion. 



It has been already remarked that the allantois is a 

 structure which extends from the body of the foetus to the 

 outer investing membrane of the ovum, that it insinuates 

 itself between the two layers of the amniotic fold, and 

 becomes fused with the outer layer, which has itself 

 become previously fused with the vitelline membrane. 

 By these means the external investing membrane of the 

 ovum, or the chorion, as it is 

 now called, represents three 

 layers, namely, the original vi- 

 telline membrane, the outer 

 layer of the amniotic fold, and 

 the allantois. 



Very soon after the entrance of 

 the ovum into the uterus, in the 

 human subject, the outer surface 

 of the chorion is found beset with 

 fine processes, the so-called villi of 

 the chorion (a, figs. 216,217), which 

 give it a rough and shaggy ap- 

 pearance. At first only cellular in structure, these little 

 outgrowths subsequently become vascular by the develop- 

 ment in them of loops of capillaries (fig. 218); and the latter 



.-,. 2lg 



