378 THE EVOLUTION OF MAN. 



the right half of the fourth arterial arch has become the 

 permanent arterial arch (arcus aorta, Fig. 301). On the 

 other hand, the latter has developed from the left half of 

 the same arch (Fig. 302) in Mammals, which are directly 

 descended from the Protamnia. 



On comparing the arterial system in the various classes 

 of the Skulled Animals (Craniota) in its matured condition, 

 it appears in very various forms, and yet it develops, in 

 all, from the same primitive form. This development takes 

 place in man exactly as in other Mammals ; especially is the 

 modification of the five arterial arches precisely the same in 

 both cases (Figs. 303-306). At first, only a single pair of 



FIGS. 303-306. Metamorphosis of the five arterial arches in the human 

 embryo (diagram after Eathke) : to, arterial stalk ; 1, 2, 3, 4, 5, the arterial 

 arches from the first to the fifth pair; ad, main stem of the aorta; aw, 

 roots of the aorta. In Fig. 303, three of the arterial arches are given ; in Fig. 

 304, the whole five (those indicated by dots are not yet developed); in Fig. 305, 

 the first two have again disappeared ; in Fig. 306, the permanent arterial 

 stems are represented. The dotted parts disappear, s, Snb-clavian artery ; 

 v, vertebral artery ; ax, axillary artery ; c, carotid artery (c', outer, c", 

 inner carotis) ; p, pulmonary artery (lung-artery). 



arches develop, and these lie on the inner surface of the 

 first pair of gill-arches (Figs. 147-150, vol. i. pp. 395-398 ; 

 Fig. 303). A second and a third pair of arches then develop 



