LATER DEVELOPMENT OF VASCULAR SYSTEM 



359 



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 

 branch from the base of the third arch 

 on each side, extending into the mandi- 

 ble and forming the external carotid ar- 

 tery. The dorsal aorta in front of the 

 third arch constitutes the beginning of 

 the internal carotid. During the fourth 

 day the sixth pair of aortic arches is 

 formed behind the fourth cleft, and the 

 origin of the pulmonary arteries is trans- 

 ferred to them (Fig. 102). The fifth pair 

 of aortic arches is also formed during the fourth day (Fig. 206.) 

 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 



Fig. 205. — Diagram of 



the aortic arches of birds 



and their fate. (After 



Boas.) 



Car. com., Common ca- 

 rotid. Car. ext., External 

 carotid. Car. int., Internal 

 carotid. D. a., Ductus ar- 

 teriosus. L., Left. p. A., 

 Pulmonary artery. P., 

 Right. 



1, 2, 3, 4, 5, and 6, First, 

 second, third, fourth, fifth, 

 and sixth aortic arches. 



