258 Vertebrate Embryology 



aortic arches, and through these to the head and fore- 

 limbs. The pulmonary trunk, arising from the right 

 ventricle, leads to the fifth pair of aortic arches, which 

 are directly continuous with the dorsal aorta of the body 

 of the embryo, and from which also the small pulmonary 

 arteries arise. From the aorta a vitelline artery carries 

 blood to the yolk-sac; and a still larger allantoic artery 

 runs from the aorta to the allantois. 



" The blood is brought back to the heart by three 

 veins: the right and left anterior venae cavae, and the 

 posterior vena cava. The right and left anterior venae 

 cavae return blood from the head and fore-limbs of the 

 embryo. The posterior vena cava returns blood from 

 the hinder part of the body, the limbs, and the kidneys; 

 just before reaching the heart it is joined by the ductus 

 venosus, which returns blood from the yolk-sac, from 

 the allantois, and from the alimentary canal of the 

 embryo, by the vitelline, allantoic, and mesenteric veins, 

 respectively. The blood in the vitelline vein is arterial 

 as regards nutrient matter; the blood in the allantoic 

 vein is arterial as regards its gaseous components; and 

 the blood in the mesenteric vein is venous. The blood 

 in the posterior vena cava is venous as regards nutri- 

 ment, and as regards gaseous components, but, having 

 just passed through the kidneys, is arterial as regards 

 freedom from nitrogenous excretory matters. 



"The blood brought to the heart by the posterior 

 vena cava may therefore be spoken of as arterial, and 

 stands in this respect in marked contrast to the venous 

 blood brought to the heart by the right and left anterior 

 venae cavse. 



" All three vense cavae open into the right auricle of 



