IX.] SUMMARY. 299 



The heart is to all intents and purposes a simple 

 twisted tube marked off by constrictions into a series of 

 three consecutive chambers. The blood coming from 

 the venous radicles passes through the heart and then 

 through the three pairs of arterial arches. 



From these it is collected into the great dorsal 

 aorta. Upon this dividing into two branches, the stream 

 of blood passes down on each side of the notochord 

 along the body, and thence out by the vitelline arteries, 

 which distribute it to the yolk-sac. 



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

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

 into the lateral trunks, of the vitelline 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 ventricles 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. 



