362 



THE DEVELOPMENT OF THE CHICK 



Cm: com 



s.cl.s 



so that its branches appear secondarily as branches of the inter- 

 nal carotid. The common carotid (car. communis) of adult 

 anatomy is derived entirely from the proximal part of the inter- 

 nal carotid. 



The Subclavian Artery. The primary subclavian artery 

 arises on the fourth day from the fifteenth (eighteenth of entire 



series) segmental artery of 

 the body-wall when the 

 wing-bud forms, and grad- 

 ually increases in import- 

 ance with the growth of the 

 wing. During the fifth day 

 a small artery that arises 

 from the base of the carotid 

 arch grows backwards and 

 unites with the primary sub- 

 clavian at the root of the 

 wing. Thus the subclavian 

 artery obtains two roots, a 

 primary one from the dorsal 

 aorta and a secondary one 

 from the carotid arch (Fig. 

 209). As the latter grows 

 in importance the primary 

 root dwindles and finally 

 disappears (about the ninth 

 day). Apparently the Cro- 

 codilia and Chelonia agree 

 with the birds in this re- 

 spect, while the other ver- 

 tebrates retain the primary 

 root. 



The Aortic System in- 

 cludes the aortic arch and 

 the primitive dorsal aorta 



Scl.d. 



'd.Ao. 



Fig. 209. — Dissection of the heart and 

 aortic arches of a chick embryo in the 

 latter part of the sixth day of incuba- 

 tion. (After Sabin.) 



Au., Auricle. Car. com., Common car- 

 otid. S'cl. d., s., primary and secondary 

 subclavian artery. 



3, 4, 6, Third (carotid), fourth (system- 

 ic), and sixth (pulmonary) arches. 



with its branches (Fig. 216). 



The segmental arteries belong to the primitive dorsal aorta; 

 originally there is a pair in each intersomitic septum, but their 

 fate has not been thoroughly worked out in the chick. At six 

 days the cervical segmental arteries are united on each side by 



