540 PROCEEDINQS OF THE NATIONAL MUSEUM vol. io4 



As Hughes has emphasized, the metamerism of the nervous, mus- 

 cular, and vascular systems serves as an aid in following changes which 

 subsequently occur during the course of embryonic development. 

 The first and second aortic arches are metamerically associated with 

 the second and thu'd pro-otic segments while the third aortic arch is 

 associated with the first post-otic segment of the early embryo. Since 

 the basal portion of the carotid arch in the adult is located at a posi- 

 tion many segments behind the auditory capsule, it is considered that 

 the aortic arches migrate posteriorly during the period of early 

 development. 



Prior to this migration, the embryo is a metamerically arranged 

 structure with the segmxcntal organs of the cephalic end in an un- 

 disturbed relationship (central nervous system with its nerve roots, 

 somites, and aortic arches). At this time segmentally arranged inter- 

 somitic arteries and veins are to be found; however, with a change in 

 this early segmental relationship and the caudad migi-ation of the 

 aortic arches, the roots of the intersomitic blood vessels become 

 severed from the aorta and these vessels then anastomose longitudi- 

 nally with one another to form the longitudinal vertebral artery. The 

 newly formed vertebral artery later acquires new connections with the 

 dorsal aorta; thus, its formation is dependent upon the posterior 

 migration of the heart and the ultimate position of the aortic arches. 



Formation of the subclavians and vertebrals are, as a result of the 

 caudad migration of the heart and aortic arches, intimately related 

 and it is likewise possible that the formation of the secondary ex- 

 ternal carotid may be closely dependent upon this same modification. 



As noted by Hughes, the third aortic arch has migrated backward 

 through 20 segments by the first half of the seventh day of incubation. 

 The carotid arch in its final position lies opposite the 15th cervical 

 ganglion, and the root of the common cervical artery (Fleming, 1926) 

 lies opposite the 18th interspace, where it joins with the persistent 

 intersomitic artery of this interspace. As a result, the distal portion 

 of the vertebral root is derived from the same position as the primary 

 subclavian artery. 



Anteriorly the vertebral artery becomes connected with the ex- 

 ternal carotid by way of a deep branch of the occipital artery which 

 runs between the occipital arch and the atlas. 



In the pig, the internal mammary artery is formed by longitudinal 

 anastomosing of the more cephalic of the thoracic intersegmental 

 arteries caudad to the subclavian artery, and subsequent deletions of 

 the proximal parts of the other intersegmentals leave it to arise from 

 the subclavian. The origin is quite similar to that of the vertebral 

 artery anterior to the subclavian. In the bird, however, the so-called 

 internal mammary (thoracic or intercostal) artery does not appear to 

 form in the same manner as in the mammal. Insofar as I can deter- 



