IX.] THE ARTERIAL ARCHES. 29] 



Fio. 92. 



^V\ 



State of Akterial Circulation on the Fifth or Sixth 

 Day. 



E.G. A. external carotid. I.C.A. internal carotid. D.A. dorsal 

 aorta. Of. A. vitelline artery. U.A. allantoic artery. 



We have already seen (Chap. vii. p. 225) that of 

 the three aortic arches which make their appearance on 

 the third day, the first two disappear : the first on the 

 fourth, the second on the fifth day ; but that their dis- 

 appearance is accompanied by the formation behind 

 them of two new aortic arches, the fourth and the fifth. 

 Thus there are generally three, never more than three, 

 pairs of aortic arches present and functional at one time. 



This statement needs some limitation ; for according to Von 

 Baer there are four arches present both on the fourth and 

 fifth days. In the case of the fourth day a sUght remnant of the 

 first pair of arches still persists when the fourth pair is already 

 formed ; and on the fifth day the second pair has not entirely 

 disappeared when the fifth pair is formed. In both of these 

 cases however the first pair of arches of the four is only present 

 for a very short time, and then is so diminished in size as to be 

 of no importance. 



]9— 2 



