DEVELOPMENT OF THE CHICK 141 



of acting as an organ of respiration. After 

 oxidation the blood is returned to the embryo 

 by veins which ultimately (at the end of the 

 third day) become continuous with the um- 

 bilical veins. These convey the blood to the 

 liver, whence it finds its way by the caudal 

 vena cava to the heart. 



Obviously the embryonic circulation differs 

 in several important respects from the circula- 

 tion in the adult. As early as the eighth day 

 of incubation a definite route has been assumed 

 by the blood. The vitelline or omphalo- 

 mesenteric veins carry blood laden with nutri- 

 ment from the yolk-sac to the liver, where it 

 is mixed with blood drained from the intestines 

 by the portal vein. Within the liver the 

 absorbed yolk doubtless undergoes some form 

 of change, but exactly what or how is not 

 clear. The right and left hepatic veins leave 

 the liver and join the caudal vena cava, which 

 opens into the right atrium of the heart. It 

 seems more than probable, however, that 

 little, if any, of the blood brought to the 

 heart by the caudal vena cava actually enters 

 the main cavity of the right atrium. Most, 

 if not all, of it passes through the foramen 

 ovale into the left atrium. From the left 



