VIII.] THE BULBUS ARTERIOSUS. 259 



cavities. The free edge of the septum of the bulbus 

 now fases with the ventricular septum, and thus the 

 division of the bulbus into two separate channels, each 

 provided with three valves, and each communicating 

 with a separate side of the heart, is complete, the po- 

 sition of the valves not being very different from what 

 it is in the adult heart. 



That division of the bulbus which opens into the 

 fifth pair of arches is the one which communicates with 

 the right ventricle, while that which opens into 

 the third and fourth pairs communicates with the left 

 ventricle (vide Fig. 93). The former becomes the pul- 

 monary artery, the latter the commencement of the 

 systemic aorta. 



The external constriction actually dividing the bul- 

 bus into two vessels does not begin to appear till the 

 septum has extended some way back towards the heart. 



The semilunar valves become pocketed at a period 

 considerably later than their first formation (from the 

 147th to the 165th hour) in the order of their ap- 

 pearance. 



Towards the end of the fifth and in the course of the 

 sixth day further important changes take place in the 

 heart. 



The venous end with its two very conspicuous au- 

 ricular appendages, comes to be situated more dorsal 

 to the arterial end, though it still turns rather towards 

 the left. The venous portion of the heart undergoes 

 on the sixth day, or even near to the end of the fifth, 

 such a development of the muscular fibres of its walls 

 that the canalis auricularis becomes almost entirely 

 concealed. The point of the heart is now directed 



17—2 



