306 THE DEVELOPMENT OF THE CHICK 
remain until hatching, and serve as a passageway of blood from 
the right side to the left side. 
The completion of the cardiac septa takes place in such a 
way that the aorta becomes connected with the left ventricle, 
the pulmonary artery remaining in connection with the right. 
To understand how this occurs it is necessary to remember that, 
although the bulbus arteriosus is primitively connected with the 
right side of the ventricle, the revolution of the latter has trans- 
ferred the bulbus to the middle line where it les to the right of 
Fic. 204. — Reconstruction of the heart of a 
chick embryo of 5.7 mm. head-length. Ven- 
tral face removed; interior of the dorsal 
half. (After Masius.) 
Atr.d.,s., Right and left auricles. D.C. 
d.,s., Right and left duets of Cuvier. E.C., 
Endothelial cushion. i. A.S., Interauricu- 
lar septum. M. V., Opening of the meatus 
venosus into the sinus. 8S. V., Sinus venosus. 
V.d.,s., Right and left ventricles. 
the interventricular septum, and ventral to the right division of 
the auricular canal. The bulbo-auricular ridge thus forms the 
floor of this side of the auricular canal. The interventricular 
septum is attached to the right side of the cushion-septum and 
its foramen and the aperture of the bulbus le side by side. It 
will also be remembered that the proximal portion of the bulbus 
is divided by a partition formed by right and left endocardial 
