DEVELOPMENT OF FOURTH DAY 



527 



arches with a fourth pair just beginning to form. It is at about this 

 period also that there is an extension headward from the dorsal aortic 

 roots. These extensions form the internal carotid arteries which supply 

 the brain. 



The external carotid arteries arise later from the ventral aortic roots. 

 They also grow cephalad as do the internal carotid arteries, but, unlike 

 the internal carotids, the external carotids supply the face. 



By the end of the fourth day two more pairs of aortic arches appear 



o 



00 



i 



2 

 J- 

 * 



il 



Fig. 309. 



Schematic diagrams illustrating the changes which take place in the aortic 

 arches. A, embryonic prototype ; B, Fishes ; C, Urodeles ; D, Lizard ; E, Birds ; F. 

 Mammals. 



The dotted lines show the portions which have become obliterated in the adult 

 forms of the animals mentioned, ao.asc., ascending aorta which branches into the 

 following aortic arches : 0,00, 1, 2, 3, 4, ; ao.desc., descending aorta ; bot, duct of 

 Botallus ; pulm, pulmonary artery ; subcl, subclavian artery ; 0,00, 1, 2, 3, and 4, 

 the six aortic arches. (After Boas.) 



posterior to those already present. The fifth pair of aortic arches is 

 very small and disappears in a short time. 



The first and second arches have become smaller and also finally 

 disappear. Probably most often the entire first arch has disappeared 

 by this time and sometimes the second has also gone. 



Consequently, there are present only the third, fourth, and sixth 

 pairs. While these arches do not remain intact permanently, though 



