258 THE FIFTH DAY. [CHAP. 



other. The existence of the septum can only be as- 

 certained at this stage by dissection or by sections, 

 there being as yet no external signs of the division. 



At the time when the septum is first formed, the 

 opening of the bulbus 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, loc. cit.) are developed from the wall of that 

 portion of the bulbus which lies between the free edge 

 of the septum and the cavity of the ventricles. 



These arise as six solid outgrowths of the wall 

 arranged in pairs, a ventral, a dorsal, and an outer pair, 

 one valve of each pair belonging to the one and the 

 other to the other of the two main divisions of the 

 bulbus which are now being established. 



The ventral and the dorsal pairs of valves are the 

 first to appear: the former as two small prominences 

 separated from each other by a narrow groove, the 

 latter as a single shallow ridge, in the centre of which 

 is a prominence indicating the point where the ridge 

 will subsequently become divided into two. The outer 

 pair of valves appear opposite each other, at a con- 

 siderably later period, between the ends of the other 

 pair of valves on each side. 



As the septum grows backwards towards the heart, 

 it finally reaches the position of these valves. One of 

 its legs then passes between the two ventral valves, 

 and the other unites with the prominence on the dorsal 

 valve-ridge. At the same time the growth of all the 

 parts causes the valves to appear to approach the heart 

 and thus to be placed quite at the top of the ventricular 



