396 



THE EVOLUTION OF MAN. 



the germ-area. Numerous blood-vessels form in the intes- 

 tinal-fibrous layer of the germ-area. They are at first 

 confined to the dark germ-area, or the so-called " vascular 



FIG. 148. Canoe-shaped 

 germ of a Dog, from the 

 ventral side ; enlarged 

 about 10 times. In front, 

 below the forehead, the 

 first pair of gill-arches are 

 visible ; below these is the 

 S-shaped bent heart, close 

 by, and on either side of 

 which lie the two ear-vesi- 

 cles. Posteriorly, the heart 

 divides into the two yelk- 

 veins, which spread them- 

 selves over the germ-area 

 (the greater part of this has 

 been torn away). At tin* 

 bottom of the open ventral 

 cavity the primitive aorta? 

 lie between the primitive? 

 vertebrae, and from which 

 five pairs of yelk-arteries 

 proceed. (After Bischotf.) 



area " (area opaca, or area vasculosa) ; but they afterwards 

 extend over the whole outer surface of the intestinal germ- 

 vesicle. The whole yelk-sac, finally, seems to be enveloped 

 in a network of blood-vessels. It is the function of these 

 blood-vessels to collect food-material from the contents of 

 the yelk-sac and carry it to the body of the embryo. This 

 is done by veins, by blood-vessels leading back, which pass 

 in at the posterior opening of the heart, first from the germ- 

 area and later from the yelk-sac. These veins are called 

 yelk -veins (venae vitellinai) ; they are also often called 

 omphalic-mesenteric veins (vence ompUalo-inesentericw}. 



