184 



EMBRYOLOGY. 



After the formation of vessels and blood is completed, the territory 

 of the area opaca, in which the processes just described take place, 

 is sharply delimited at its periphery (fig. 117) in all meroblastic eggs, 

 as well as in those of Mammals. HV>r the dose network of blood- 



vessels ends abruptly at its periphery in a broad, circular, marginal 

 vein (the vena or sinus terminalis, S.T.). 



Beyond the sinus terminalis, there is formed on the yolk neither 

 blood nor blood-vessels. Nevertheless, the two primary germ-layers 

 spread themselves out laterally over the yolk still farther, the 

 outer layer more rapidly than the inner, until they have grown 

 entirely around it. 



