THE HEART. 415 



wall of the auricular chamber, and grows down into its cavity 

 (cf. Fig. 163, RD). For a time the lower and posterior edge of 

 the auricular septum is free, but during the fourteenth day it 

 meets and fuses with a cushion-like thickening of the margin 

 of the auriculo-ventricular aperture. 



Before this fusion is completed, however, a new aperture, the 

 foramen ovale, is formed in the dorsal and anterior part of the 

 auricular septum, through which free communication between 

 the two auricles is maintained up to the time of birth of the 

 young rabbit. 



The pulmonary veins develop rather late, and are of small 

 size until near the time of birth ; the two veins, from the right 

 and left lungs respectively, unite to form a single vessel, which 

 opens into the dorsal wall of the left auricle, very close to the 

 auricular septum. 



The ventricular portion of the heart. The ventricular cavity 

 is at first single, and receives the blood from the auricular cavity 

 through a transverse slit in its dorsal wall. 



The division of the ventricular cavity into right and left 

 ventricles is effected by a septum, which grows upwards from 

 the apex of the ventricle towards the auriculo-ventricular aper- 

 ture. This ventricular septum (cf. Fig. 163) appears about the 

 twelfth day, and its position is indicated from an early period by 

 a groove on the surface of the heart. The septum remains in- 

 complete for some time, the two ventricles communicating above 

 its free edge. About the fifteenth day the septum meets, and 

 unites with, the cushion-like thickenings of the margin of the 

 auriculo-ventricular aperture, and so completes the separation 

 between the two ventricles. 



The thickening of the wall of the ventricle is effected in the 

 first instance, just as in the frog, by the ingrowth of muscular 

 trabecules into the cavity, which unite to form a reticulum (cf. 

 Fig. 163), the proper wall of the ventricle remaining thin. 

 In the later stages, however, the outer walls of the ventricles 

 thicken considerably throughout their entire substance. Up to 

 the time of birth there is practically no difference in thickness 

 between the walls of the right and left ventricles, the resistance 

 to be overcome being approximately the same in the two 

 cases. 



