142 THE STRUCTURE OF THE FOWL 



atrium it enters the left ventricle, and thence 

 is pumped into the aorta and supplies the 

 body by arteries corresponding to those of the 

 adult. The aorta of the embryo, however, 

 gives off omphalo-mesenteric arteries, which carry 

 blood to the yolk-sac — ^there to receive a fresh 

 supply of nutriment — and umbilical arteries, 

 conveying blood to the allantois in order that 

 it may receive oxygen and get rid of carbon 

 dioxide. 



The blood carried by the carotid and sub- 

 clavian arteries to the head, wing, and side of 

 the thorax is returned to the right atrium 

 by the two cranial venae cavae. From the 

 atrium it passes into the right ventricle and 

 so into the pulmonary trunk, which divides 

 into right and left pulmonary arteries. The 

 lungs, however, being as yet without function, 

 admit of the passage of very little blood. 

 Almost all the blood of the pulmonary artery 

 is transferred into the aorta through a con- 

 nection with this vessel known as the ductus 

 arteriosus. 



Purified blood is returned to the embryo 

 from the allantois by an umbilical vein — the 

 left of the two veins originally present — which 

 joins the left hepatic vein. Clearly the purest 



