588 GENERATION AND DEVELOPMENT. 



Development of Bloodvessels. 



At an early period of development, and during the changes 

 just described, an accumulation of cells ensues between the 

 mucous and serous laminae at a part of the germinal mem- 

 brane named the area vasculosa (b, Fig. 218). Within this 

 mass, which constitutes a third or middle layer of the blasto- 

 dermic membrane, is laid the foundation for the development 

 of the vascular system. At the circumference of the vascular 

 area, insulated red spots and lines make their appearance, and 

 these soon unite, so as to form a network of vessels filled with 

 blood. The margin of the vascular layer is at first limited and 

 quite circular, being bounded by vessels united in a cireulus 

 venosus, or sinus terminalis, 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 bloodvessels are developed originally from nu- 

 cleated cells. These cells sent out processes ; the processes 

 from different cells unite ; and in this way ramifications 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. 599). 



The Chorion. 



It has been already remarked that the allantois is a structure 

 which extends from the body of the foetus to the outer in- 

 vesting membrane of the ovum, that it insinuates itself be- 

 tween the two layers of the amniotic fold, and becomes fused 

 with the outer layer, which has itself become previously 



fused with the vitelline membrane. 

 FIG. 225. By these means the external in- 



vesting membrane of the ovum, or 

 the chorion, as it is now called, 

 represents three layers, namely, the 

 original vitelline 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. 223, 224), which 

 give it a rough and shaggy ap- 

 pearance. At first only cellular in 



