750 



GENERATION AND DEVELOPMENT. 



It must not be supposed that the phenomena which have 

 "been successively described, occur in any regular order one 

 after another. On the contrary, the development of one 

 part is going on side by side with that of another. 

 Fig. 223. Fig. 224.* 



Development of Blood-vessels. 



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. 2 1 8). Within this 

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

 dermic membrane, is laid the foundation for the develop- 

 ment 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 circulm venosus, or sinus terminate, 



* Figs. 223 and 224 (after Todd and Bowman), a, chorion with 

 villi. The villi are shown to be best developed in the part of the 

 chorion to which the allantois is extending ; this portion ultimately 

 becomes the placenta, b, space between the two layers of the amnion. 

 c, anmiotic cavity, d, situation of the intestine, showing its connexion 

 with the umbilical vesicle. e } umbilical vesicle. /, situation of heart 

 and vessels, y, allantois. 



