640 THE HEART OF AVES. 



It commences on the third day as a crescentic ridge or fold springing 

 from the convex or ventral side of the rounded ventricular portion of the 

 heart, and on the fourth day grows rapidly across the ventricular cavity 

 towards the concave or dorsal side. It thus forms an incomplete longitu- 

 dinal partition, extending from the canalis auricularis to the commencement 

 of the truncus arteriosus, and dividing the twisted ventricular tube into 

 two somewhat curved canals, one more 

 to the left and above, the other to 

 the right and below. These commu- , 



nicate with each other, above the free 

 edge of the partition, along its whole 

 length. 



Externally the ventricular portion 

 as yet shews no division into two parts. 



By the fifth day the venous end of 

 the heart, though still lying somewhat 

 to the left and above, is placed as far FIG. 360. HEART OF A CHICK ON 



forwards as the arterial end, the whole THE FOURTH DAY OF INCUBATION 



VIEWED FROM THE VENTRAL SURFACE. 



organ appearing to be drawn together. 



The ventricular septum is complete. ** Mt . auricular appendage; C.A. 



- , . . . canalis auricularis : v. ventricle ; b. trun- 



The apex of the ventricles becomes cus arteriosus. 



more and more pointed. In the au- 

 ricular portion a small longitudinal fold appears as the rudiment of the 

 auricular septum, while in the canalis auricularis, which is now at its greatest 

 length, there is also to be seen a commencement of the valvular structures 

 tending to separate the cavity of the auricles from those of the ventricles. 



About the io6th hour, a septum begins to make its appearance in the 

 truncus arteriosus in the form of a longitudinal fold, which according to 

 Tonge (No. 495) starts at the end of the truncus furthest removed from the 

 heart. It takes origin from the wall of the truncus between the fourth and 

 fifth pairs of arches, and grows downwards in such a manner as to divide the 

 truncus into two channels, one of which leads from the heart to the third and 

 fourth pairs of arches, and the other to the fifth pair. Its course downwards 

 is not straight but spiral, and thus the two channels into which it divides 

 the truncus arteriosus wind spirally the one round the other. 



At the time when the septum is first formed, the opening of the truncus 

 arteriosus into the ventricles is narrow or slit-like, apparently in order to 

 prevent the flow of the blood back into the heart. Soon after the appearance 

 of the septum, however, semilunar valves (Tonge, No. 495) are developed 

 from the wall of that portion of the truncus which lies between the free edge 

 of the septum and the cavity of the ventricles 1 . 



1 If Tonge is correct in his statement that the semilunar valves develop at some 

 distance from the mouth of the ventricle, it would seem possible that the portion of 

 the truncus between them and the ventricle ought to be regarded as the embryonic 

 conus arteriosus, and that the distal row of valves of the conus (and not the proximal 

 as suggested above, p. 639) has been preserved in the higher types. 



