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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 'princifalis). 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 about 

 five times). The posterior end of the heart {d), -vvhich is curved in the form 

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

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

 the circular boundary vein, or terminal vein (r. 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) (p). The dark area which surrounds the head 

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

 (After BischoEf.) ^ 



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