398 THE EVOLUTION OF MAN. 



embryo. The vessels passing from the heart in front are the 

 gill-arch arteries, leading from the heart, and which, rising 

 as aorta-arches, encircle the anterior end of the intestine, 

 and unite in the main aorta (aorta principalis). The two 

 branches, which result from the division of this main artery, 



FIG. 150. Embryo and germ-area of a Rabbit, in which the first system 

 of blood-vessels is complete, seen from the ventral side (magnified aboujb 

 five times). The posterior end of the heart (d), which is curved in the form 

 of an S, divides into two large yelk-veins, each of which sends out an 

 anterior branch (b) and a posterior branch (c). The ends of these unite in 

 the circular boundary vein, or terminal vein (v. terminalis) (a). In the germ- 

 area may be seen the coarser venous network (lying below), and the finer 

 arterial network (lying nearer the surface). The yelk-arteries (/) open 

 into the two primitive aortae (e). The dark area which surrounds the head 

 like a halo, represents the recess within the head-cap or membrane. 

 (After Bischoff.) 



