Development of the Fifth Day 269 



changes in the heart from this time until the 

 time of hatching will now be mentioned. 



During the fifth day the interventricular 

 septum is completed, and becomes fused an- 

 teriorly with the posterior edge of the septum 

 that is developed at this time, in the truncus 

 arteriosus. This latter septum is attached, 



r. 



FIG. 82. HEART OF A CHICK ON THE SIXTH DAY 



OF INCUBATION, VENTRAL SURFACE. (After Foster 



and Balfour.) 



/., left auricular appendage, r.a, right auricular append- 

 ge. r.v^ right ventricle. /.z>, left ventricle. 



anteriorly, between the fourth and fifth pairs 

 of aortic arches, and follows a sort of spiral 

 course backwards, to where it joins the inter- 

 ventricular septum. The effect of these septa 

 in directing the blood from the right and left 

 ventricles into different channels has already 

 been explained (page 244). Before the com- 

 pletion of the septum of the truncus arteriosus, 



