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Fig. 72. Stages in the differentiation of the parts of the heart. Ventral view. 

 (A) Elasmobranch. (B) Teleost. (C) Amphibian. (D) Lower reptile. (E) Alligator. 

 (F) Bird and mammal, (a) Auricle; (ao) aorta; (b) bulbus arteriosus; (c) conus; (cd) 

 duct of Cuvier; (/;) hepatic veins; (pa) pulmonary artery; (pc) pre- and postcaval 

 veins; (pv) pulmonary vein; (s) sinus venosus; (sa) septum atriorum; (») ventricles. 

 (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, 

 The Blakiston Company.) 



Fig. 73. Diagrams to show the evolution of the aortic arches. Ventral views. 

 (A) Primitive condition with six aortic arches. (B) Fishes, the first aortic arch 

 missing. (C) Some urodeles, the first, second, and fifth arches missing. (D) Anurans, 

 with the connection (k) between the pulmonary arteries (d) and the aorta oblit- 

 erated. (E) Reptiles, showing the ventral aorta split into three trunks and the 

 fourth aortic arch (// and i) persistent on both sides. (F) Birds, the ventral aorta 

 split into two trunks, and the fourth aortic arch (/() persistent on the right side 

 only. (G) Mammals, the ventral aorta split into two trunks and the fourth aortic 

 arch (i) persistent on the left side only, (a) Interruption of the aortic arches by the 

 gill capillaries in fishes; (b) internal carotid; (c) external carotid; (d) pulmonary, 

 developed from the sixth arch; (c) ventral aorta; (/) dorsal aorta; (</) aortic arch; 

 (h) right fourth aortic arch; (7) left fourth aortic arch; (J) common carotid; (k) arte- 

 rial ligament, a vestige of the vessel originally connecting pulmonary and aorta; 

 (I) subclavian. (Modified from Wilder. Courtesy, Hyman: "Comparative Verte- 

 brate Anatomy," University of Chicago Press.) 



