THE PATH OF DEGENEEATION IN ANIMALS 191 



weeks old shows the presence of a series of slits on 

 the sides of the neck, the slits not being parallel, 

 but converging towards the ventral surface. Be- 

 tween these slits are swellings, or pads, which" pass 

 up towards the dorsal surface and appear like the 

 beginnings of hoops or ribs enclosing the visceral 

 cavity ; the elevations are the branchial arches, the 

 slits are the gill-slits. 



In the human embryo (fig. 66, A), as in fish, 

 these slits appear 

 from above 

 downwards, and 

 as they are 

 formed, the cor- 

 responding blood- 

 vessels arise. 



These vessels, 

 or aortic arches, 



(n \ FIG. 66. Diagram of branchial arches in mammals. 



(0.) A Embryonic stage, a, aorta; c, aortic arches; 



crivpcj nflf -^ A dorsal aorta. B. Adult stage. The parts 



& iv represented by dotted lines have degenerated. 



six lateral ^> aorta ? v > carotid; ap, pulmonary artery. 



branches (c.) at each side. These lateral branches 

 run up between the gill- slits and form two main 

 trunks on the dorsal side which converge to form 

 the descending aorta (ad.). 



In man the branchial arches are transformed, 

 parts of them entering into the structure of the 

 face, and during the transformation parts of the 

 aortic arches atrophy (fig. 66, B). But the order 



arise from a Ven- 



tral 



