300 THE SIXTH DAY. [CHAP. 



The blood from the yolk-sac and allantois, together 

 with a small quantity from the intestine, is collected 

 into the portal vein, and by that vessel carried to the 

 liver. Here it becomes divided into two streams, part 

 flowing directly by the ductus venosus into the sinus 

 venosus, and the remainder passing through the capil- 

 laries of the liver, being brought back to the ductus 

 venosus by the hepatic veins. 



During this period the blood is aerated both by the 

 allantois and yolk-sac, but as yet chiefly by the latter. 



At a somewhat late period of incubation the 

 blood from the ventricles passes into two entirely dis- 

 tinct roots. The one of these, that from the right 

 chamber, sends the blood to the fifth pair of arches; 

 passing through which the greater part of the blood 

 flows into the dorsal aorta, a small portion only finding 

 its way into the lungs through the as yet unimportant 

 pulmonary arteries. 



Through the other aortic root, viz. that from the 

 left ventricle, the blood flows into the third and fourth 

 pairs of arches. That part of the blood which flows 

 into the third pair, passes almost entirely to the head 

 and upper extremities by the external and internal 

 carotids ; that which flows into the right arch of 

 the fourth pair is chiefly brought to the dorsal aorta, 

 but some of it passes to the right wing ; that, on the 

 contrary, which goes into the left fourth arch is for the 

 most part sent to the left wing, a small part only reach- 

 ing the dorsal aorta. There is still a mixture of the 

 blood from the two chambers of the heart, so that the 

 blood in the dorsal aorta is composed partly of blood 

 from the left, and partly from the right chambers. 



