IX.] THE PULMONARY ARTERIES. 298 



single median aorta terminating behind in the caudal 

 artery (Figs. 92, 94). The arteries to the kidneys, 

 hind limbs, etc. are developed as branches of this aorta. 



As the allantois grows rapidly and becomes an im- 

 portant respiratory organ, the allantoic or umbilical 

 arteries increase in size. As a general, though ap- 

 parently not invariable rule, the right allantoic artery 

 gets gradually smaller and soon disappears. 



The vitelline artery (Of. A) now leaves the aorta 

 as a single but quickly bifurcating trunk, which at the 

 end of the fifth day is still very large. 



By the fifth day the ventricular portion of the heart 

 (compare Chap. VII. p. 257) is completely divided into 

 two chambers. The bulbus arteriosus is also divided 

 by a septum into two channels, one of which com- 

 municates with the right ventricle of the heart and the 

 other with the left. 



One result of this arrangement is that all the 

 blood which passes to the anterior extremity of the 

 body comes from the left ventricle of the heart. 



At about the seventh day an entire separation 

 begins to take place between the arterial roots which 

 come respectively from the right and left chambers of 

 the heart. The root from the right chamber (Fig. 93) 

 remains connected with the fifth pair of arches. The 

 root from the left ventricle is connected with the third 

 and fourth pairs of arches. 



The lower part of the body still receives blood from 

 both the right and left ventricles, since the blood which 

 enters the fifth arch still flows into the common dorsal 

 aorta. As the lungs however increase in size, a com- 

 munication is set up between them and the fifth pair of 



