DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 255 



of the truncus arteriosus. The aperture from the ventricle to the 

 truncus arteriosus is guarded by pocket valves. 



The truncus arteriosus may as before be divided into a proximal 

 part or pylangium which receives the blood from the ventricle, and a 

 distal part or synangium which divides on each side into the afferent 

 branchial vessels. 



The pylangium and synangium are separated from each other 

 by well-formed valves. Of these there were two at the stage last 

 described, in the form of right and left flaps, placed transversely to 

 the vessel. In place of these two simple valves, there are now three 

 pocket valves, with free upwardly-directed edges. These valves are 

 very unequal in size, and consist of a large one on the right side of 

 the truncus, and two smaller ones, anterior and posterior, on the 

 left side. They correspond in the order given to the valves num. 

 bered by Boas* 1, 2, and 3 respectively. 



Below these valves the cavity of the pylangium is single, but is 

 partially divided by the spiral valve. This valve has the same 

 relations as before. It is a longitudinal inwardly projecting fold 

 of the pylangium, which, commencing at the ventricular aperture, 

 where it lies to the left of the pocket valves, runs up somewhat 

 spirally along the posterior or dorsal wall of the pylangium, and 

 ultimately to its right side, where it ends immediately below the 

 right hand valve, valve No. 1 of Boas, of the three between pylan- 

 gium and synangium. The condition is therefore practically that of 

 the adult, except that the upper end of the spiral valve, though 

 lying just below valve No. 1, has not yet fused with it. 



The distal part of the truncus, or synangium, is now divided 

 internally by a vertical partition into anterior and posterior vessels, of 

 which the anterior divides on each side to form the first and second 

 branchial afferent vessels, while the posterior similarly divides to 

 form the third and fourth branchial afferent vessels. In the synan- 

 gium itself this division is an absolute one, the first and second arches 

 being shut off completely from the third and fourth. The septum 

 which effects this division ends in a free lower edge immediately 

 above the three pocket valves that separate the synangium from the 

 pylangium. 



* Unas, " I 'i-li'-r <l'Ti Conns Arteriosus und die Aortenbogen der Amphibien," 

 ''Jlorphologjschefl Jabrbucb," vii., 1881, ]>p. 502-507, and plate xxiv,, Pigs. 15 and 10. 



