396 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



names dorsal and ventral (Maekay, 1889) are to be preferred to internal 

 and external, for the latter though in common use are less precise. 



During the ' development of the young individual there is laid 

 down a general scheme of aortic arches and associated vessels 

 agreeing with that just described, and the processes of modification 

 whereby there becomes evolved out of this the complicated and 

 very different arrangement of the great arteries of the adult afford 

 materia] for one of the most fascinating chapters in vertebrate 



an. d.c V.c. 



Fig. 187. — Illustrating modification of the carotid arteries, correlated with elongation 



of the neck region. 



A, Varanid Lizard ; B, Grass-snake (Tropidonotus) ; p.c, primary carotid. 

 (Other letters as in Fig. 1S6.) 



embryology. The general lines of these processes are best illustrated 

 by an outline of what happens in the group Reptilia. 



The arrangement which the main arteries assume in adult 

 Reptiles shows much variety. The relation which the adult 

 arrangement in the more important Reptilian types bears to the 

 primitive scheme may be gathered from an inspection of Figs. 186, C, 

 187, .A, B, and 187a, C. 1 Through the conspicuous differences in 



1 It must be remembered that in the actual animal the various bends and turns 

 of the vessels tend to become straightened out. For example arch III becomes 

 simply a portion of a straight internal carotid artery. In the diagrams the original 

 curvature of the arches is retained for the sake of clearness. 



