V.] THE AORTIC ARCHES. 121 



last two disappear in the chick without giving rise to any permanent 

 structures. 



The first visceral cleft remains permanently open, but is drawn out by the 

 orowth of surrounding parts into a long tube, subsequently divided into the 

 meatus auditorius and the Eustachian tube. The other visceral clelts are 

 obliterated. 



15. By the end of the second day three pairs of aortic 

 arches had been established in connection with the heart. 

 When the visceral folds and clefts are formed, a definite 

 arrangement between them and the aortic arches is always 



Fig. 38. 



The same Head as shewn in Fig. 37, seen from the Front. 



The neck has been cut across between the first and second visceral folds, the 

 incision being carried throusrh the first visceral cleft. In the cut surface b 

 are seen the sections of the hind brain H b., and of blood-vessels c. 



I F. The first visceral fold ; between the ends of the fold is seen a section of 

 the somatopleure at its extreme forward limit ; in it lies the aorta a. 

 Below the folds is the cavity of the throat al, and /. Fis placed in the first 

 visceral cleft. 



C.H. Cerebral hemispheres. iV. Nasal pit. ch.f. groove indicating the 

 choroidal fissure. 



observed. The first visceral cleft runs between the first and 

 second aortic arches. Consequently the first aortic arch runs 

 in the first visceral fold and the second in the second. In 

 the same way, the second visceral cleft lies between the 

 second and third aortic arches, the third aortic arch running 

 in the third visceral fold. Each aortic arch runs in the 

 thickened mesoblast of the corresponding fold. 



Arrived at the upper surface of the alimentary canal, 

 these arches unite at acute angles to form a common trunk, the 

 dorsal aorta (Fig. 39 A, A. 0), which runs along the back imme- 

 diately under the notochord. The length of this common single 

 trunk is not great, as it soon divides into two main branches 



