484 EMBRYOLOGY OF THE CHICK 



which to draw the oxygen that permeates through the egg shell and the 

 albumen surrounding the yolk. 



After the yolk has been absorbed as food-material, and the blood 

 has become oxygenated, the blood is collected into the sinus terminalis 

 and the vitelline veins. The latter converge toward the embryo from 

 all parts of the vascular layer, and empty into the omphalomesenteric 

 veins, which return the blood to the heart. 



The blood which has been sent to the various parts of the embryo 

 has in the meantime been returned from the head region by the anterior 

 cardinals, and from the caudal end by the posterior cardinals, the an- 

 terior and posterior cardinal of each side having met to form a short 

 common cardinal (duct of Cuvier) through which the blood flows into 

 the sinus portion of the heart. 



There is therefore a mixed circulation in the heart, consisting of 

 both embryonic and extra-embryonic blood. The extra-embryonic, of 

 course, is the richer in both food and oxygen supply. 



THE EXCRETORY SYSTEM 



After about ten somites' have been formed, the beginnings of the 

 excretory system are visible. 



It will be remembered that the mid-region of the partially seg- 

 mented mesoderm, known as the mesomere, is to become the excretory 

 system. It can be noted first as a solid cord of cells extending for two 

 or three somites (Fig. 285). This will be called the Wolffian Duct as 

 soon as a lumen forms. 



During the second half of the second day, this solid rod elongates 

 both headward and tailward, the more tailward portion becoming free 

 and lying between ectoblast and mesoblast. A lumen appears toward 

 its center and extends headward and tailward simultaneously. About 

 the beginning of the fourth day the duct definitely opens into the cloaca. 



The Wolffian body also makes its appearance on the second day, but 

 it will be better understood if the description is reserved until later. 



