PRIMITIVE CIRCULATORY SYSTEM. 



397 



Thus the first circulatory system of the blood in the 

 embryo (Figs. 148-150) occurs in all the higher classes of 



Fig. 149. — Embryo and germ -area of a Rabbiv, in which, the earliest 

 rudiments of the blood-vessels appear,- -seen from the ventral side (magni- 

 fied about ten times). The posterior end of the simple heart (a) divides 

 into two large yelk-veins, which form a network of blood-vessels on the 

 dark germ-area (which appears light on the black background). At the 

 head extremity the fore-brain with the two eye- vesicles (bb) may be seen. 

 The dark centre of the germ is the wide-open intestinal cavity. Ten 

 primitive vertebrae are visible on each side of the notochord. (After 

 Bischoff.) 



Vertebrates in the following simple order. The very simple 

 pouch-shaped heart (Fig. 150, d) divides both in front and 

 behind into two vessels. Those at the back are veins 

 leading to the heart. They take food-material from the 

 germ- vesicle, or yelk- sac, and carry it to the body of the 



