LATER DEVELOPMENT OF VASCULAR SYSTEM 349 
left one first at an early stage and the right one later. The left 
auricle is thus larger than the right for a considerable period of 
time in the early development. When the right auricle grows 
out it passes above the bulbus, which is already in process of 
rotation, and the two auricles then expand ventrally on each 
side of the bulbus. The apex of the ventricle belongs primarily 
to the left side and this remains obvious as long as the external 
interventricular groove exists. In the adult the apex of the 
heart belongs to the left ventricle. 
Fic. 199.— Ventral view of the heart of a 
chick embryo of 5 mm. head-length. 
(After Masius.) 
Atr. d., s., Right and left auricles. 
B. Co. Bulbus cordis. V. Ventricle. 
The varying positions occupied by the chambers of the heart in rela- 
tion to the body axes constitute a serious difficulty in describing the 
development. For instance, the auricular canal is at first in front of 
the atrium (before any bending of the heart takes place). As the ven- 
tricular loop turns backward and beneath the atrium, the auricular 
‘anal is ventral to the atrium; and finally, as the ventricles assume their 
definitive position behind the auricles, the derivatives of the auricular 
canal (auriculo-ventricular openings) come to lie behind the atrium. In 
other words, the atrium rotates around a transverse axis through nearly 
180 degrees in such a way that its original anterior end becomes succes- 
