THE BLOOD VESSELS. 



477 



it is fairly closely adherent to the yolk. This uplifted portion 

 of the gut is the ' fore-gut.' 



Examine a series of sections, or selected sections, of a two- 

 day-chiclc. Draw and name the parts. 



Fig. 88. Transverse section across the head of a Chick Embryo at the 

 forty-third hour of incubation. The section is taken immediately 

 behind the auditory pits and the heart, and passes through the 

 rudiments of the glosso-pharyngeal nerves, x 100. 



A, aorta. BH, cavity of hind-brain. CH, notochord. E, epiblast. h, 

 hypoblast. ME, somatopleuric layer of mesoblast. MH, splanchnopleuric 

 layer of mesoblast. n B, neural crest : the part shown in the figure gives rise 

 later on to the ganglia of the glosso-pharyngeal nerves. VV, vitelline vein. 



In the region of the somites the undivided mesoblast is 

 during the second day differentiated to form the embryonic 

 kidney or mesonephros (fig. 90, KC). This arises from the 

 intermediate cell-mass as described on p. 482. 



The Blood-vessels. 



The vascular system on the second day consists of (1) 

 paired vitelline veins bringing nutritious blood from the yolk 

 to the heart ; (2) tubular heart ; (3) paired aorta dividing 

 into : (4) vitelline arteries. In addition there is a well- 

 developed circulation in the capillaries of the yolk-sac. The 

 veins of the embryo have scarcely begun to develop. 



KK 



