236 PROFESSOR MARSHALL AND EDWARD J. BLES. 



part of the truncus just before it divides into right and left branches ; 

 these folds are equal in size, and placed opposite each other, pro- 

 jecting into the cavity of the truncus from the right and left sides 

 respectively ; the aperture between them, leading to the distal part 

 of the truncus, is very small. 



The proximal part of the truncus arteriosus, or pylangium, is 

 partially divided by a longitudinal fold projecting into its cavity. 

 This longitudinal fold, which becomes the longitudinal valve of the 

 adult, arises below from the left side of the ventral lip of the ven- 

 tricular aperture, and runs up the truncus in a spiral manner, passing 

 round to the posterior or dorsal wall and then to the right side. Its 

 upper or distal end is in very close relation with the right valve of the 

 pair described above as marking the separation between the proximal 

 and distal parts of the truncus, and at a slightly later stage it fuses 

 with this right valve. 



The longitudinal valve appears first in tadpoles of about 11 mm. 

 length. 



Beyond the paired valves the truncus arteriosus divides almost at 

 once into right and left branches (Fig. 14). Each of these runs 

 directly outwards, and speedily divides into three branches, of which 

 the two anterior ones become the afferent vessels for the first and 

 second branchial arches, while the third runs backwards and out- 

 wards, and then divides into the afferent vessels for the third and 

 fourth branchial arches. 



It may be noted that although valves are present in the truncus 

 arteriosus, the two primary branches of the truncus are still 

 undivided, so that it is difficult to imagine that the valves play 

 any part in regulating the passage of the blood along the several 

 afferent vessels. It is, however, significant that these valves should 

 appear just at the time when the auricular septum is forming, and 

 the lungs are coming into use. 



4. The Blood Vessels of the Visceral Arches (Figs. 13 

 and 14). 



a. The Mandibular Arch. — As in the earlier stages, there is no trace 

 of an afferent vessel belonging to this arch. The diverticulum of the 

 dorsal aorta, which we have described in the previous stage as the 

 mandibular artery (Fig. 11, E M), is readily recognised at 12 mm., 



