302 THE (muck. 



•101' 



venosus and the pulmonary vein, and ends in a free poster 

 edge. Of the two cavities into which it partially divides fche 

 auricular chamber, the left auricle is for a time much the larger 

 of the two. 



The interventricular septum also appears during the fourth 

 day, as a crescentic partition which arises from the ventral wall 

 of the apex of the ventricular chamber, and gradually extends 

 across towards the dorsal wall. It divides the ventricular 

 chamber somewhat obliquely, and as yet imperfectly, into a left 

 and more dorsally placed cavity, and a right and more ventrally 

 placed one. The position of the septum is indicated by a slight 

 groove on the surface of the heart. 



On the fifth day the interventricular septum is completed, 

 but the interauricular septum remains imperfect throughout the 

 whole period of development, up to the time of hatching. 



The septum of the truncus arteriosus appears on this day as a 

 longitudiual fold, corresponding exactly to the similar one in the 

 frog. The fold commences near the distal end of the truncus 

 arteriosus, between the fourth and fifth pairs of aortic arches, 

 and grows backwards with a somewhat spiral course, dividing the 

 cavity of the truncus arteriosus into right and left halves. By 

 the end of the fifth day this longitudinal septum has grown 

 back to the base of the truncus arteriosus, and now meets with 

 the upper edge of the interventricular septum and fuses with 

 this. The eifect of this fusion is that the right ventricle now 

 communicates with the right division of the truncus arteriosus, 

 and through this with the hindmost or fifth pair of aortic arches 

 alone ; while the left ventricle communicates, through the left 

 division of the truncus arteriosus, with the anterior pairs of 

 aortic arches, but no longer with the fifth or hindmost pair. 



Before the completion of the septum of the truncus arteriosus 

 three semilunar valves are formed at the base of each of the 

 divisions of the truncus arteriosus, between these and the 

 ventricles. 



During the sixth day the shape of the heart as a whole 

 approaches much more closely to that of the adult ; the apex of 

 the heart becomes more pointed, and the auricular appendices 

 more prominent. 



