I90~ THE FIFTH DAY. [CHAP. 



widely and form the 'sinus rhomboidalis,' which is not, as 

 has been sometimes stated, the remains of the primitive 

 'sinus rhomboidalis' visible during the second day. 



The anterior white columns have much increased on this 

 day, and now form the sides of the already deep anterior 

 fissure. The anterior white commissure does not however 

 appear till somewhat later. 



7. The fifth day may perhaps be taken as marking a 

 most important epoch in the history of the heart. The 

 changes which take place on that and on the sixth day, 

 added to those previously undergone, transform the simple 

 tube of the early days of incubation into an almost com- 

 pletely formed heart. 



The venous end of the heart, though still lying somewhat 

 to the left and above, is now placed as far forwards as the 

 arterial end, the whole organ appearing to be drawn together. 

 The ventricular septum is complete. 



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 longitu- 

 dinal fold, which according to Dr Tonge {Proc. of Royal Soc. 

 1868) starts, not (as Von Baer thought) at the end of the 

 bulbus nearest to, but at that furthest removed from, the 

 heart. It takes origin from the wall of the bulbus between 

 the fifth and fourth pairs of arches and grows downwards 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 downwards 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 downward:^ 

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

 which it divides the bulbus arteriosus, wind spirally the one 

 over the other. The existence of the septum can only be 



