LATER DEVELOPMENT OF VASCULAR SYSTEM 309 
carotid. The third arch on each side persists as the proximal 
portion of the internal carotids; and the dorsal aorta ruptures 
on each side between the dorsal ends of the third and fourth 
arches. The fourth arch and the root of the dorsal aorta dis- 
appear on the left side, but remain on the 
right as the permanent arch of the aorta. 
The fifth arch disappears on both sides; 
the sixth arch persists throughout the 
period of incubation and forms an im- 
portant arterial channel of the systemic 
circulation until hatching. Then the 
dorsal portion (duct of Botallus or duc- 
tus arteriosus) becomes occluded, and 
the remainder of the sixth arch becomes 
the proximal portion of the pulmonary 
arteries. 
The details of these changes are as 
follows: On the third and fourth days of 
incubation the first and second aortic 
arches disappear (Fig. 102). The lower 
ends of these arches then appear as a Fig. 205. — Diagram of 
branch from the base of the third arch the aortic arches of birds 
on each side, extending into the mandi- and their fate. (After 
Boas. ) 
es ae i Car. com., Common ca- 
tery. The dorsal aorta in front of the rotid. Car. ext., External 
third arch constitutes the beginning of carotid. Car. int., Internal 
: ; : : is earotid. D.a., Ductus ar- 
the internal carotid. During the fourth  teriosus. L., Left. p. A., 
: : 3 : ‘ > PS ESP aoe 
day the sixth pair of aortic arches is SS apa artery. R., 
cae P : : ight. 
formed behind the fourth cleft, and the 1, 2, 3, 4, 5, and 6, First, 
origin of the pulmonary arteries is trans- second, third, fourth, fifth, 
ar, a : ma es : and sixth aortic arches. 
ferred to them (Fig. 102). The fifth pair 
of aortic arches is also formed during the fourth day (Fig. 206.) 
ble and forming the external carotid ar- 
It is a slender vessel passing from near the base to near the 
summit of the sixth arch. As it has been entirely overlooked 
by most investigators, it is certain that it is of very brief duration. 
and it may even be entirely absent in some embryos. Apparently 
it has no physiological importance, and it can be interpreted only 
as a phylogenic rudiment. 
Thus at the beginning of the fifth day the entire series of 
aortic arches has been formed, and the first, second, and fifth 
