208 



CHORDATE ANATOMY 



to notochord and dorsal aorta. Furthermore, their segmentation is 

 independent of that of the visceral arches. Another point of resemblance 

 is that the first and second head cavities divide during ontogenesis into 

 dorsal and ventral moieties precisely as do the first and second post-otic 



myotomes in Petromyzon. The fusion of portions of two myotomes, the 

 second and the third, to form the external rectus muscle of the eye resem- 

 bles the fusion of trunk myotomes such as occurs in the formation of the 

 tongue muscles. 



