CIRCULATORY SYSTEM OF THE GRASS-SNAKE. 



619 



The Bidbus cordis, as lias been pointed out above, is not to be 

 found as a separate structure in the adult, and so the three aortic 

 arches arise directly from the ventricle. The base of each of 

 these is guarded by two semilunar valves, which Langer (29) has 

 shown to be homologous with the distal row of valves in the 

 amphibian heart. 



III. The Arterial System. (PI. LXX.) 

 (A) Beveloptnent. 



The development of the aortic arches in Tropidonotios is very 

 similar to that of other Reptilia*. It was first described by 

 Rathke (30), whose general account has been confirmed since by 

 Van Bemmelen (7 & 8) except in one particular. Rathke 



Text-fiff. 88. 



Diagram of embrj^onic arterial arches. It represents the condition after the 

 disappearance of arches 1, 2, and 5, and shows also the division of the truncus 

 arteriosus into three arterial roots. 



C.A. Common carotid. D.A. Dorsal aorta. E.G. External carotid. I.C. Internal 

 carotid. L.S.A. Left systemic arch. P.A. Pulmonary artery. P.Ar. Pulmonary 

 arch. li.S.A. Right systemic arch. 



Adapted from Hochstetter (22). 



describes the development of only five visceral arches on each 

 side, which he numbered from the anterior end 1-5, Yan Bem- 

 melen, however, showed that there was another arch, which has 

 however, a somewhat transient existence, between arches 4 and 5 

 of Rathke, so that the latter's fifth arch is in reality the sixth of 

 the series, and thus the snake is brought into line with other 

 Amniota. 



* For a general account of this see Hochstetter (22). 



41* 



