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i9 8 Vertebrate Embryology 



should be one more finger, to represent the 

 hindermost arch and cleft, but as the hinder 

 arches and clefts form no part of the adult 

 chick, this omission is of little consequence. 



Further details in the building up of the 

 face and head will be given later. 



The vascular system. By the end of the 

 second day, as has already been said, there 

 are two or three pairs of aortic arches present, 

 which carry the blood from the bulbus arte- 

 riosus around the pharynx to the dorsal aorta. 



When, on the third day, the visceral folds 

 and clefts become established, they bear a 

 very definite relation to the aortic arches. The 

 first aortic arch lies in the first or mandibular 

 fold ; the second arch lies in the second or 

 hyoid fold ; the third arch in the third fold, 

 etc.; each arch lies in its corresponding fold 

 (Fig. 63, AH\ and is separated from the ad- 

 jacent arches by the visceral clefts. 



The heart, during this day, becomes still 

 more twisted, and in cross-sections of the 

 chick it is seen, usually, as two large cavi- 

 ties (Fig. 69, Ci, C$ ) under the body of the 

 dorsal region. It is relatively enormously 

 large, and lies, as yet, entirely outside of 

 the body-cavity (Fig. 57, C). The ventrally 



