SECONDARY CIRCULATORY SYSTEM. 399 



the primitive aortse, send out right and left the yelk-arteries, 

 which leave the body of the embryo and pass into the germ- 

 area. Here, and in the circumference of the navel-vesicle, 

 two layers of vessels are distinguishable — the superficial 

 arterial layer, and the lower venous layer. The two are 

 connected together. At first this system of blood-vessels 

 is extended only over the superficial front of the germ-area 

 as far as the edge. Here, on the edge of the dark vascular 

 area, all the branches unite in a large terminal vein (vena 

 terminalis, Fig. 150, a). This vein disappears at a later 

 period, as soon aS; in the course of development, the for- 

 mation of blood-vessels progresses further, and then the 

 yelk-vessels traverse the whole yelk-sac. When the navel- 

 vesicle degenerates, these vessels, of course, also degenerate, 

 being of importance only in the first period of embryonic 

 life. 



This first circulation in the yelk-sac is replaced, at a 

 later period, by the second circulation of blood in the 

 embryo, that of the allantois. Large blood-vessels are 

 developed on the wall of the primitive urinary sac, or 

 allantois, from the intestinal-fibrous layer. These vessels 

 grow larger and larger, and are most intimately connected 

 with the vessels that develop in the body of the embryo 

 itself. This secondary allantois circulation thus gradually 

 takes the place of the original, primary, yelk-sac circulation. 

 When the allantois has grown to the inner wall of the 

 chorion, and has changed itself into the placenta, its blood- 

 vessels alone accomplish the nourishment of the embryo. 

 They are called navel-vessels (vasa umhilicalia), and are 

 originally in pairs : one pair of navel arteries, and one pair 

 of navel veins. The two navel-veins (vence umhilicales, 



