VI 



AETEEIAL SYSTEM 



401 



conus and its derivatives, with the right side of the veutricular 

 cavity in proximity to the opening of the pulmonary artery. With 

 the separation of the two ventricles the arch in question remains 

 connected with the right ventricle. With increasing efficiency of 

 the pulmonary circulation the venous blood of the right ventricle 

 would be drawn off more and more to the pulmonary artery and, 

 correlated with this, we find in those Sauropsida in which metabolism 

 is most active and respiration most efficient that this fourth 

 arch on the left side with its aortic root disappears completely 

 during development, leaving only the single right-hand arch and 



n m 



a.cw 



Fn ;. 



189. — Blood-vessels of Crocodile of stage 55-56. (After Hochstetter, 19 

 Arteries are shown in outline, veins black. 



a.c. c, anterior cardinal vein ; A.r, aortic root ; &.a, basilar artery ; d.c, dorsal carotid ; ot, otocyst ; 

 p.c.v, posterior cardinal vein ; r.p, right pulmonary artery \ v.c, ventral carotid ; III-VT, aortic arches. 



root to form the proximal part of the systemic aorta (Birds, Fig. 

 187a, D). 



Arch V, in the Amniota, appears only transiently and so greatly 

 reduced in size as to have completely escaped the notice of the 

 earlier investigators. Hence in Eathke's classical scheme of the 

 aortic arches which is given in the older text-books only five arches 

 are shown, the posterior one being called the fifth. With our 

 present-day knowledge of .the homology of the lungs of Amniota 

 with those of Crossopterygians and Lung-fishes, such a scheme is 

 clearly erroneous, as it would involve the pulmonary artery, which 

 is certainly the same vessel throughout, taking its origin in the 

 Amniotes from the fifth and in the Ichthyopsida from the sixth 

 aortic arch. So without any special embryological data we should 



VOL. ii 2d 



