THE LATER DEVELOPMENT OF THE FROG 181 



sally with the lateral dorsal aorta, forming thus, before the 

 gills appear, a continuous aortic arch in each branchial arch 

 (Fig. 64, A). There are, therefore, in the branchial arches, four 

 pairs of aortic arches; these are really the third to sixth pairs 

 of aortic arches, the first and second being formed in the 

 mandibular and hyoid visceral arches. 



When the external gills appear an additional vessel develops 

 dorso-laterally to the aortic arch, along the base of the gill, 

 forming its supply. This vessel opens out of the ventral end 



KFio. 65. Diagrams of the aortic arch of the adult frog and tadpole. After 

 aurer. A. The continuous aortic arch of the adult; showing the parts corre- 

 . onding with the larval vessels. B. First external gill and associated vessels 

 in young tadpole. C. Internal gill and associated vessels in the tadpole after 

 the disappearance of the external gills, ab, Afferent branchial artery; e, epi- 

 thelioid body; eb, efferent branchial artery; eg, external gill; ig, internal gill; 

 operculum; x, direct anastomosis between afferent and efferent branchial 



e : 



f the aortic arch and joins it again toward its upper end 

 (Fig. 64, B); the lower end of the aortic arch may then be 

 termed the afferent branchial artery, its dorsal end the efferent 

 branchial artery. The small vessels of the external gills form 

 loops connecting the dorsal and ventral parts of this second 

 vessel. Then as the external gills disappear and the internal 

 gills develop on the branchial arches, the direct ventral con- 

 nection between the original aortic arch and the second vessel, 

 becomes interrupted by the disappearance of a part of the 

 aortic arch, and the vascular networks of the internal gills 

 connect the two vessels. In this way the original aortic arch 

 becomes almost entirely the efferent branchial artery, while 

 the second vessel serves as the afferent branchial artery 

 (Figs. 64, 65). 



