136 EARLY EMBRYOLOGY OF THE CHICK 



in the rich plexus of small vessels on the yolk, the blood finally 

 makes its way either directly into one or another of the larger 

 vitelline veins, or to the sinus terminalis which acts as a collecting 

 channel, and then over the sinus terminalis to one of the vitel- 

 line veins. The vitelline veins conVerge toward the yolk-stalk 

 where they empty into the omphalomesenteric veins. The 

 omphalomesenteric veins at first paired throughout their 

 entire length h^ve been brought together proximally by the 

 closure of the ventral body wall and become fused to form a 

 median vessel within the^ body of the embryo. It is through 

 this vessel that the vitelline blood eventually reaches the 

 heart. In the heart the blood of the vitelb*ne,intra-embryonic, 

 and allantoic circulations is mingled. The mixed blood passes 

 out by the ventral aorta and the aortic arches into the dorsal 

 aorta. Leaving the dorsal aorta through the vitelline arteries 

 the blood is returned to the yolk-sac. 



It should not be inferred that the blood stream ''picks up" 

 deutoplasmic granules and carries them to the embryo. The 

 acquisition of food material by the blood depends on the activ- 

 ities of the entodermal cells fining the yolk-sac. These cells 

 secrete digestive enzymes which break down the deutoplasmic 

 granules. The liquified material is then absorbed by the yolk- 

 sac cells and transferred to the blood. The blood carries the 

 food material in soluble form to the embryo where it is finally 

 assimilated. 



The Allantoic Circulation. — The allantoic arteries arise by 

 the prolongation and enlargement of the segmental vessels 

 arising from the aorta at the level of the allantoic stalk. Their 

 size increases rapidly as the allantois increases in extent. From 

 them the blood is distributed in a rich plexus of vessels which 

 ramify in the mesoderm of the allantois (Fig. 47). 



The situation of the allantois directly beneath the porous 

 shell is such that the blood can carry on interchange of gases 

 with the outside air (Fig. 30, D). It is in the rich plexus of 

 small allantoic vessels where the surface exposure is very great 

 that the blood gives off its carbon dioxide and takes up oxygen. 



At a later stage of development the ducts of the embryonic 

 excretory organs open into the allantoic stalk near its cloacal 

 end. As the excretory organs become functional the allantoic 

 vesicle becomes the repository for the nitrogenous waste mate- 



