CIRCULATION 



66 1 



aortae. The system they form changes into vessels of the adult 

 as described on pp. 600-604. The blood which the allantois 

 receives comes from the dorsal aorta by a pair of allantoic 

 arteries (Fig. 515, C al.a.). After being oxygenated in the organ 

 this blood passes by the allantoic vein (Fig. 515, D, al.v.), through 



n.f 

 Httl- 





W 



^t) V:: 





:-^i.>:.:..:..-.--'.:-!-<./-^;<€i^'"'v; 

 '•■■ ' T/« ^ I'-': "i^" 



^IM«V-^, 



B ^ 



Fig. 516. — Semi-diagrammatic views of the process of formation of the heart in 



the chick. — Partly after Patten. 



A. Two separate endothelial tubes with thickenings of the adjacent coelomic wall for the heart muscles. 

 B. The endothelial tubes are brought together, and aortic arches begin to grow out from their anterior 

 ends (around the pharvnx to join the dorsal aorta). C. The tubes unite (the muscle rudiments join 

 around them) and thus' the heart is formed. Indications of its S flexure (p. 659) are akeady visible. 



a.a., Aortic arches ; c, ccelom ; eth., endothelial lining (endocardium) of heart ; f.b., fore-brain ; h., heart ; 

 h.m., muscular wall of heart ; n./., neural folds ; ph., future pharynx ; vit.v., left vitelline vein. Compare 

 Fig. 511. 



the portal system into the inferior vena cava. In the heart, so 

 long as the chick is breathing by its allantois, the blood brought 

 to the right auricle by the inferior vena cava is not sent into the 

 right ventricle, but, by a crescentic fold — known as the Eustachian 

 valve — of the auricular wall, is directed through the foramen 

 ovale, an opening in the partition between the auricles. Thus 

 the arteriaUsed blood from the allantoic vein, mixed, it is true, 

 with some venous blood from other constituents of the inferior 



