VII.] THE HEART. 229 



a single trunk, the allantoic vein, which, running along 

 the splanchnopleure, falls into the vitelline vein (Fig. 

 74, U.). 



Meanwhile the heart is undergoing considerable 

 changes. Though the whole organ still exhibits a 

 marked curvature to the right, the ventricular portion 

 becomes directed more distinctly ventralwards, forming 

 a blunted cone whose apex will eventually become the 

 apex of the adult heart. 



The concave (or dorsal) walls of the ventricles be- 

 come much thicker, as did the convex or ventral walls 

 on the third day. 



Well-marked constrictions now separate the ven- 

 tricles from the bulbus arteriosus on the one hand, and 

 from the auricles on the other. The latter constriction 

 is very distinct, and receives the name of canalis av/ri- 

 cularis (Fig. 75, G.A.); the former, sometimes called 

 the fretum Halleri, is far less conspicuous. 



Fia. 75. 



Heart op a Chick on the Fourth Day of Incubation 

 VIEWED from the Vbntral Sueface. 



La. left auricular appendage. G.A. canalia auricularis. v. ven- 

 tricle, b. bulbus arteriosus. 



