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 



Fro. 148. Canoe-shaded 

 germ o f 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 the 

 bottom of the open ventral 

 cavity the primitive aorta? 

 lie between the primitive 

 vertebrae, and from which 

 five pairs of yelk-artcrics 

 proceed. (After Bischoff.) 



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 (vence vitellincK); they are also often called 

 omphalic-mesenteric vein's (vence omphalo-mesentericce'). 



