VIII.] THE VENTEICULAR SEPTUM. 257 



transform the simple tube of the early days of in- 

 cubation into an almost completely formed heart. 



The venous end of the heart, though still lying 

 somewhat to the left and dorsal, is now placed as far 

 forwards as the arterial end, the whole organ appearing 

 to be drawn together. The ventricular septum is com- 

 plete. 



The apex of the ventricles becomes more and more 

 pointed. In the auricular 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 commencing 

 transverse partition tending to separate the cavity of 

 the auricles from those of the ventricles. 



About the 106th hour, a septum begins to make its 

 appearance in the bulbus arteriosus in the form of a 

 longitudinal fold, which according to Tonge (Proc. 

 of Royal Soc. 1868) starts, not (as Von Baer thought) 

 at the end of the bulbus nearest to, but at that farthest 

 removed from, the heart. It takes origin from the wall 

 of the bulbus between the fifth and fourth pairs of 

 arches and grows backwards in such a manner as to 

 divide the bulbus into two channels, one of which leads 

 from the heart to the fourth and third pair of arches 

 and the other to the fifth pair. The free edge of the 

 septum is somewhat V-shaped, so that its two legs as 

 it were project backwards towards the heart, further 

 than its central portion ; and this shape of the free 

 edge is maintained during the whole period of its 

 growth. Its course backwards is not straight but 

 spiral, and thus the two channels into which it divides 

 the bulbus arteriosus wind spirally the one over the 

 F. &B. 17 



