VIII.] THE CIRCULATION ON THE FIFTH DAY. 221 



From these it is collected into the sfreat dorsal aorta. 

 On this dividing into two branches, the stream of blood also 

 divides and passes down on each side of the notochord along 

 the body, and thence out by the omphalo-mesaraic arteries, 

 which distribute it to the yolk-sac. 



In the yolk-sac it partly passes into the sinus terminalis 

 and so into the fore and aft trunks, partly directly into the 

 lateral trunks, of the omphalo-mesaraic veins. In both cases 

 it is brought back to the two venous radicles and so to the 

 heart. 



On this day the blood is aerated in the capillaries of the 

 yolk-sac. 



On the fifth or sixth day, the two auricles are present 

 though having a common cavity. The septum of the ven- 

 tricles is nearly complete, so that the blood on entering the 

 ventricles from the auricles is divided into two streams. 

 These two streams pass respectively from the right and left 

 chambers of the heart into the two divisions of the bulbus 

 arteriosus. The blood from the right ventricle passes into 

 the fifth pair of arches and that from the left ventricle into 

 the third and fourth pairs of arches. 



From the anterior parts the blood is brought back by the 

 anterior cardinal or jugular veins ; from the hinder parts of 

 the body, chiefly by the cardinal veins, but also in part by 

 the now commencing vena cava inferior. 



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 capillaries of the liver and being brought 

 back to the sinus 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 distinct roots. The 

 one of these, that from the right chamber, sends the blood to 

 the fifth pair of arches. Passing through these two arches, 

 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. 



