760 THE CIRCULATORY SYSTEM 



The transformation of the aortal arches in the shark, frog, chick, and 

 mammal is shown in figures 341 and 342. It is important to observe that, 

 in those vertebrates possessing lungs, the pulmonary artery grows back from 

 the sixth aortal arch. In a sense, however, the pulmonary arteries represent 

 a direct caudal growth from the posterior ventral aortae, particularly in rep- 

 tiles, birds, and mammals (fig. 342A, B, C, E, F, G). 



5. Dorsal Aortae (Aorta) and Branches 



Two dorsal aortae arise first, one on either side of the notochord and 

 above the primitive gut tube, and their origin is synchronous with the forma- 

 tion of the ventral, vitelline (subintestinal) blood vessels and the heart. Pos- 

 terior to the pharyngeal area, the primitive dorsal aortae soon fuse to form 

 a secondary vessel, the dorsal aorta, lying below the notochord. Anteriorly, 

 in the pharyngeal area, they remain separate, and the cephalic end of each 

 primitive dorsal aorta grows forward into the developing forebrain area. 

 These forward growths of the primitive dorsal aortae into the forebrain area 

 form the anterior rudiments of the internal carotid arteries. The primitive dorsal 

 aortae, therefore, give origin to a single secondary vessel, the dorsal aorta, 

 which is bifurcated at its cephalic end in the region of the pharyngeal area 

 of the gut. 



Aside from the cephalic ends of the internal carotid arteries, three main 

 sets of arteries arise from the developing dorsal aorta: 



1 ) Dorsal intersegmental arteries, passing between the somites and send- 

 ing a dorsal branch toward the neural tube and epaxial musculature 

 and a lateral branch into the hypaxial musculature (fig. 343A). The 

 lateral branches develop into intercostal and lumbar arteries of the 



Fig. 341. Modifications of the aortal arches. In the following diagrams, the aortal 

 arches are depicted in such a way as to represent two parts, viz. an afferent system, con- 

 veying the blood from the heart to the branchial (gill) region, and an efferent system, 

 leading the blood away from the branchial area. The afferent system of vessels is finely 

 stippled, whereas the efi^erent system is ringed with lines. With the exception of certain 

 lateral views all diagrams have been made from the dorsal view. (A-D) Aortal vessel 

 changes in embryos of Squahis acunfhias. (A and B, adapted from actual conditions 

 described by Scammon, 1911. See reference under Fig. 339.) (A) Generalized, basic 

 condition present in embryo of 15 mm. embryo. (B) Lateral view, 20.6 mm. stage. 

 (C and D) The afferent and efferent systems in the adult form. D should be superim- 

 posed upon C. Diagrams C and D have been separated to minimize confusion. (E-G) 

 Modifications of the aortal arches in the frog. The modifications of the aortal arches 

 in the frog involve a complicated series of changes. In Fig. 335 (A) the simple tubular 

 aortal arches are shown during the earlier phases of development. In Fig. 257 (B) a 

 later stage is depicted. In the latter figure the aortal arches are separated into functional 

 afferent and efferent vessels supplymg the branchiae or gills. At the time of meta- 

 morphosis the vessels are reorganized, apparently, into tubular vessels according to the 

 pattern shown in Fig. 341 (E). The transformations of the basic conditions shown in 

 Fig. 341 (E) into the adult form are outlined in Figs. 341 (F and G). (H) The 

 three divisions of the bulbus cordis in the turtle. 



