internal carotid 



dorsal aorta 



(.external carotid 



truncus arteriosus'^ 



pulmonary artery 



A B 



Figure 11-12. Lateral views of two stages of aortic arch develop- 

 ment in the chick. (After Lillie, 1919) 



the carotid arch (Figure 11-11 D). The limb lies somewhat 

 anterior to the heart in the adult bird. Again the crocodilian 

 parallels this change. 



submental 



external carotid interior carotid 



6th arch (incomplete 

 in some specimens) 



C^ musculocutaneous 



pulmonary artery 



dorsal oorta_. 



coeliaco-mesenteric artery 



A CRYPTOBRANCHUS 



B FROG 



Figure 11-13. Aortic arches of Cryptobranchus, A, and Rono, B, as 

 seen in semidiagrommatic ventral view. 



Amphibians 



Among the amphibians bilateral arches are retained. A 

 well-developed fifth arch is observed in Cryptobranchus (Fig- 

 ure 11-13). Ventrally the sixth arch is small. It gives rise to 

 the pulmonary artery, then, in most urodeles but not in the 

 frogs, continues to the dorsal stem as the ductus arteriosus. 



The pulmonary artery bears a musculocutaneous branch. 

 In Necturus the first e-xternal gill is served by the third afferent 

 arch; the efferent arch extends down and forward from the 

 gill as the external carotid (Figure 11-14). Cryptobranchus is 

 similar even though an external gill is lacking. Neither has a 

 carotid body. The base of the sixth arch is lost in Necturus; the 



internal carotid 



dorsal aorta 



ductus arteriosus ^ ~y^^ 



lung 



pulmonary vein 



external carotid 



internal gill ''ventral aorta 



A URODELA, LARVA 



ductus caroticus (when open) 

 / ductus arteriosus 



B NECTURUS 



carotid gland 



truncus if. „»,;,,„ 



lett atrium 



C TYPICAL URODELE ADULT (SALAMANDRA) D ANURA, ADULT 



Figure 11-14. Fate of aortic arches in different amphibions. A, lorvol urodele; B, Necturus,- 

 C, typical adult urodele (So/amondro); D, adult onuron. (After Goodrich, 1930) 



THE AORTIC ARCHES • 349 



