THE AORTIC ARCHES 179 



The aortic arches 



The aortic arches become modified with development, and these changes 

 are outlined below in tabular form. Stages in the development of the chick 

 embryo are designated either by time in hours from the beginning of incuba- 

 tion or by the number of mesodermal somites present. The somites form as 

 discrete, paired structures, and the first pair appears at about 20 hours. The 

 number of pairs of somites increases from 1 at 20 hours to 41 at 96 hours. 

 In references to the number of pairs of somites the word "pairs" is usually 

 dropped and thus a 10-somite embryo means 10 pairs, not 5 pairs. 



The Aortic Arches 



Ultimate fate in 

 later development 

 break down at 72 hr. ; form the exter- 

 nal carotid arteries 



break down at 96 hr. ; form the exter- 

 nal carotid arteries 



break down at 7 days; form the inter- 

 nal carotid arteries 



right side forms main systemic artery; 

 left side breaks down 

 transitory; break down at 5 days 

 form the pulmonary arteries; remain 

 attached to systemic arteries until hatch- 

 ing 



In the table an arch is said to break down and form other arteries. Actu- 

 ally the vessel constricts and the blood stops flowing through the arch. The 

 vessel then parts at the constriction and the two ends are free to grow into 

 adjacent tissues and become new arteries. Thus, for example, when the first 

 and second aortic arches become severed from the dorsal aorta their ventral 

 portions remain connected with the ventral aorta and grow forward as the 

 external carotid artery. (See Figures 102 and 106.) 



Although the aortic arches can be of no respiratory value to the embryo, 

 we see that they are changed into important arteries in the adult. Of particu- 

 lar interest is the survival of the right aortic arch as the main artery of the 

 heart. In mammals the right aortic arch breaks down and it is the left arch 

 which persists. 



