TAXONOMY, ANATOMY, EMBRYOLOGY 



Figure 21. Diagrams of the Aortic Arches and Their Derivatives in a Series of 

 Vertebrates. A, hvpothetical ancestor; B, shark; C, the kingfish Protoptenis; D, tele- 

 ost; E, salamander; F, Hzard; G, bird; H, mammal. Roman numerals indicate aortic 

 arches; s, spiracle; Arabic numerals, gill slits; cd, carotid duct; da, ductus arteriosus; 

 ec, external carotid arterv; ic\ internal carotid arterv; L, lung. (From Romer, "The 

 Vertebrate Body," 2nd Ed., W. B. Saunders Co., Philadelphia, Pa., 1955. ) 



early in development, but it soon becomes continuous again. The dorsal 

 connection between arches three and four disappears, with the result that 

 the ventral connections now appear as common carotid arteries on either 

 side, while the two fourth arches now supply the major circulation to the 

 body. The fifth arch becomes reduced in size and may be lost altogether, 

 while the sixth arch again gives rise to a pulmonary branch. In the Anura 

 and in the reptiles, this process goes a little farther, with the fifth arch 

 being lost completely and with the dorsal part of the sixth arch being lost, 

 so that all of the blood entering the sixth arch must go into the pulmonary 

 artery. The birds have essentially the reptilian system, with, however, the 

 left fourth aortic arch degenerating, thus leaving the right fourth arch 

 to carrv the entire svstemic circulation. In the mammals, it is the rieht 

 fourth arch which degenerates and the left one which persists. Thus, of 

 the six original pairs of aortic arches, only three persist in the highest 



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