THE AORTIC AECHES. 305 



aortic arches acquire definite relations with the latter ; the first, 

 or most anterior aortic arch of each side (Figs. 113 and 124, Ai), 

 lying in the first or mandibular arch ; the second aortic arch, 

 A2, lying in the hyoid arch ; and the third aortic arch, A3, in 

 the first branchial arch. During the fourth day, fourth and 

 fifth pairs of aortic arches (Fig. 128, A4, AS) appear in the 

 second and third branchial arches. 



There are thus altogether five pairs of aortic arches in the 

 chick, corresponding to the five anterior of the six pairs present 

 in the tadpole. These arches, however, differ from those of the 

 tadpole inasmuch as (i) they never have any gills developed in 

 connection with them ; (ii) they form from their first appearance 

 direct connections between the truncus arteriosus and the aorta, 

 there being no separation into afferent and efferent vessels. A 

 further difference lies in the fact that the aortic arches in the 

 mandibular and hyoid arches are complete in the chick, while 

 they never become so in the tadpole. 



The condition of the aortic arches in the chick is comparable 

 to that of a frog after the metamorphosis ; but it is at present 

 a matter of doubt whether this indicates an entire omission of 

 the earlier stages in the chick, owing to the absence of gills ; 

 or whether the continuous aortic arch from heart to aorta does 

 not rather represent a still earlier ancestral condition, prior to 

 the acquisition of gills. 



From the five pairs of aortic arches of the embryo the adult 

 arterial system is derived, in the following manner. 



During the fourth, or fourth and fifth days, the first or 

 mandibular, and the second or hyoidean aortic arches disappear 

 along the middle portions of their lengths ; their ventral and 

 dorsal portions persist, the ventral portions remaining of com- 

 paratively small size as the lingual or mandibular arteries 

 (Fig. 128, AL); while the dorsal portion, which is much larger, 

 extends forwards into the head as the carotid artery (Fig. 128, 

 AC), which divides into internal and external carotid arteries, 

 supplying the brain and face respectively. 



By the end of the fifth day the ventricular septum is com- 

 pleted, and has fused with the longitudinal or spiral septum of 

 the truncus arteriosus (cf. p. 302). This latter septum arises, 

 in front, between the roots of the fourth and fifth pairs of aortic 



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