292 



ANIMAL PHYSIOLOGY. 



ing, and removed from the inner layer or yelk sac by an 

 increasing amount of fluid, rises up round the whole embryo, 

 as it has previously risen round the head; and thus the 

 embryo is dipped into a deepening hollow, and at last the 

 entrance into the hollow becomes narrowed and obliterated 

 above the dorsum of the embryo, and the embryo is com- 

 pletely enveloped in a sac, which is filled with fluid, and is 

 called the amnion. The part of the outer layer of the ger- 

 minal membrane beyond the amnion becomes incorporated 

 with the chorion. The amnion continues, till birth, as a 

 perfectly transparent membrane filled with clear liquor 

 imnii, in which the foetus lies. 



Fig. 156. FCETAL CONNECTIONS: diagram, a, Sac of the amnion; 

 b, yelk sac ; c, allantois becoming developed into the f cetal part 

 of the placenta. 



213. We have still to consider the development of the 

 Vascular system. The first traces of embryonic blood-vessels 

 seem to make their appearance external to the embryo, in the 

 opaque border of the clear area which surrounds it. Here 

 there is a circular vena terminalis, and a development of blood- 

 corpuscles. A network of vessels springs up over the clear 

 area, and the blood is brought to the embryonic heart by two 

 trunks, which enter the embryo transversely, one on each 

 side, the omphalo-mesenteric veins. The circular vena ter- 

 minalis advances on the inner layer of the germinal mem- 

 brane, until the network of vessels of which it forms the 

 limit, the ompkalo-mesenteric system, surrounds the whole 

 yelk sac. This is the earliest system of blood-vessels for the 

 nourishment of the embryo; and in the eggs of oviparous 

 animals, which are comparatively large, it is of great import- 



