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, 



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

 of blood-vessols is complete, — seen from tlie ventral side (magnified about 

 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 aorta) (e). The dark area which surrounds the head 

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

 (After Bischoff.) 



