VII.] THE ARTERIAL ARCHES. 225 



It is not, however, till the second half of the fourth day, 

 when the allantois grows rapidly, that these allantoic y 

 or, as they are sometimes called, umbilical, arteries 

 acquire any importance, if indeed they are present 

 before. 



The vitelline arteries are before the end of the day 

 given off from the undivided aortic trunk as a single 

 but quickly bifurcating vessel, the left of the two 

 branches into which it divides being much larger than 

 the right. 



During this day, the arterial arch running in the 

 first visceral fold becomes obliterated, the obliteration 

 being accompanied by the appearance of a new (fourth) 

 arch running in the fourth visceral fold on either 

 side. 



The second pair of arterial arches also becomes 

 nearly (if not entirely) obliterated; but a new pair of 

 arches is developed in the last (fifth) visceral fold, 

 behind the last visceral cleft; so that there are still 

 three pairs of arterial arches, which however now run 

 in the third, fourth and fifth visceral folds, the last of 

 these being as yet small. If we reckon in the slight 

 remains of the second pair of arches we may consider 

 that there are in all four pairs of arches. When the 

 first and second arches are obliterated, it is only the 

 central portion of each arch on either side which abso- 

 lutely disappears. The ventral portion connected with 

 the bulbus arteriosus, and the dorsal portion which 

 joins the dorsal aorta, both remain, and are both carried 

 straight forward towards the head. The ventral por- 

 tions of both first and second arches unite on each side 

 to form a branch, the external carotid (Fig. 73, E.C.A.), 

 F. & B. 15 



