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. 



Fig. 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 

 canal 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- 



