362 THE DEVELOPMENT OF THE CHICK 
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- 
Fic. 209. — Dissection of the heart and 
aortic arches of a chick embryo in the ; 
latter part of the sixth day of incuba- Spect, while the other ver- 
tion. (After Sabin.) tebrates retain the primary 
Au., Auricle. Car. com.,Common car- oot, 
otid. Sel. d., s., primary and secondary : 5 
subclavian artery. The Aortic System in- 
3, 4, 6, Third (carotid), fourth (system- cludes the aortic arch and 
ic), and sixth (pulmonary) arches. Sia Pea 
the primitive dorsal aorta 
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 
