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TEXT-BOOK OF EMBRYOLOGY. 



appear on each side. The first, second, third, and fourth pass through the 

 corresponding branchial arches. The fifth arch, which is merely a loop from 

 the fourth, seems to pass through the fourth branchial arch. The sixth aortic 

 arch passes through the region behind the fourth branchial. All these arches are 

 present in embryos of 5 mm. (Fig. 218). In Fishes and larval Amphibians, where 

 the branchial arches develop into the gills, the aortic arches are broken up into 

 capillary networks which ramify in the gills, and the ventral aortic root becomes 

 the afferent vessel, the dorsal aortic roots the efferent vessels. In the higher 

 Vertebrates and in man the aortic arches begin, at a very early period, to 

 undergo changes; some disappear and others become portions of the large 



Vent, aortic roots 



VI 

 Dorsal aortic roots 



Aorta 



Subclavian arteries 



FIG. 219. Diagram of the aortic arches of a Mammal. Modified from Hochstetter. 



arterial trunks which leave the heart. In connection with the following 

 description, constant reference to Figs. 219 and 220 will assist the student in 

 understanding the changes. 



The first and second arches soon atrophy and disappear. The third 

 arch on each side becomes the proximal part of the internal carotid artery, while 

 the continuation of the dorsal aortic root, cranially to the third arch, becomes 

 its more distal part. The continuation of the ventral aortic root cranially to the 

 third arch, becomes the proximal part of the external carotid artery, while the 

 portion of the ventral aortic root between the third and fourth arches becomes 

 the common carotid artery. The portion of the dorsal aortic root between the 



