354 THE DEVELOPMENT OF THE CHICK 
closed, but, as we shall see later, the interventricular foramen 
is utilized in connecting up the aorta with the left ventricle. 
It will be seen that if the original direction of this septum, 
as indicated by the interventricular groove on the surface, were 
preserved (Fig. 199), the interventricular septum would fuse 
with the bulbo-auricular ridge and the right ventricle would then 
be continuous with the bulbus only, and the left ventricle with 
the atrium, and circulation of the blood would be impossible. 
The avoidance of this condition is due to the rotation of the bul- 
bus by which it is brought beneath the auricular canal, and by 
widening of the auricular canal to the right. Thus the inter- 
Fig. 202. — Frontal section of the heart of a chick 
embryo of 9 mm. head-length. (After Hochstet- 
ter.) 
E. C., Median endothelial cushion. 1. E. C., Lat- 
eral endothelial cushion. $8. Atr., Septum atriorum. 
S. V., Septum ventriculorum. 
ventricular septum meets the right side of the cushion-septum 
and divides the auricular canal, though the opening of the bulbus 
remains on its right. 
(c) The interauricular septum forms at the same time as the 
septum between the ventricles, as a thin myocardial partition 
arising from the vault of the atrium between the openings of the 
sinus venosus and pulmonary vein; it extends rapidly with con- 
cave free border towards the auricular canal, and soon fuses 
