DEVELOPMENT. 691 



their appearance successively, so that there are five pairs in all, each one 

 running along one of the visceral arches. 



These five are never all to be seen at once in the embryo of higher 

 animals, for the two anterior pairs gradually disappear while the poste- 

 rior ones are making their appearance, so that at length only three 

 remain. 



In Fishes, however, they all persist throughout life as the branchial 

 arteries supplying the gills, while in Amphibia three pairs persist 

 throughout life. 



In Keptiles, Birds, and Mammals, further transformations occur. 



In Reptiles the fourth pair remains throughout life as the perma- 

 nent right and left aorta; in Birds the right one remains as the perma- 

 nent aorta, curving over the right bronchus instead of the left as in 

 Mammals. 



i<iG. 477. Fio. 478. 



FIG. 477. Diagram of young embryo and its vessels, showing course of circulation in the um- 

 bilical vesicle; and also that of the allantois (near the caudal extremity), which is just commencing. 

 (Dalton.) 



FIG. 478. Diagram of embryo and its vessels at a later stage, showing the second circulation. 

 The pharynx, oesophagus, and intestinal canal have become further developed, and the mesenteric 

 arteries have enlarged, while the umbilical vesicle and its vascular branches are very much, re- 

 duced in size. The large umbilical arteries are seen passing out hi the placenta. (Dalton.) 



In Mammals the left fourth aortic arch develops into the permanent 

 aorta, the right one remaining as the subclavian artery of that side. 

 Thus the subclavian artery on the right side corresponds to the aortic 

 arch on the left, and this homology is further confirmed by the fact that 

 the recurrent laryngeal nerve hooks under the subclavian on the right 

 side and the aortic arch on the left. 



The third aortic arch remains as the internal carotid artery, while 

 the fifth disappears on the right side, but on the left forms the pulmo- 

 nary artery. The distal end of this arch originally opens into the de- 

 scending aorta, and this communication (which is permanent through- 

 out life in many reptiles on both sides of the body) remains throughout 



