78 EARLY EMBRYOLOGY OF THE CHICK 



extra-embryonic plexus. Later some of these channels become 

 confluent, others disappear, and gradually definite main vessels, 

 the omphalomesenteric arteries, are established. For some 

 time after their formation, the omphalomesenteric arteries are 

 likely to retain traces of their origin from a plexus of small 

 channels and arise from the aorta by several roots (Fig. 35). 



The Beginning of the Circulation of Blood.— At about 44 

 hours of incubation, coincident with the completion of the 

 vitelline vessels, the heart begins regular contraction, and the 

 blood which has been formed in the extra-embryonic vascular 

 * area is for the first time pumped through the vessels of the 

 embryo. In tracing the course of either the embryonic or the 

 vitelline circulation the heart is the logical starting point. 

 (_From the heart the blood of the extra-embryonic vitelline circu- 

 lation passes through the ventral aortae, along the dorsal aortae, 

 and out through the omphalomesenteric arteries to the plexus 

 V_jof vessels on the yolk. 



In the small vessels which ramify in the membranes envelop- 

 ing the yolk the blood absorbs food material. In young 

 embryos, before the allantoic circulation has appeared, the 

 vitelhne circulation is involved also in the oxygenation of the 

 blood. The great surface exposure presented by the multitude 

 of small vessels on the yolk makes it possible for the blood to 

 take up oxygen which penetrates the porous shell and the 

 albumen. 



After acquiring food material and oxygen the blood is 

 collected by the sinus terminahs and the vitelhne veins. The 

 vitelline veins converge toward the embryo from all parts of 

 the vascular area and empty into the omphalomesenteric veins 

 which return the blood to the heart (Fig. 48). 



The blood of the intra-embryoriic circulation, leaving the 

 heart enters the ventral aortae, thence passes into the dorsal 

 aortae, and is distributed through branches from the dorsal 

 aortae to the body of the embryo. It is returned from the 

 cephaHc part of the body by the anterior cardinals, and from 

 the caudal part of the body by the posterior cardinals. The 

 anterior and posterior cardinals discharge together through the 

 ducts of Cuvier into the sinus region of the heart (Fig. 24) . 



In the heart, the blood of the extra-embryonic circulation 

 and of the intra-embryonic circulaSon is mixed. The mixed 



