HISTORY OF THE EYE MUSCLES 



449 



not differentiate muscle fibers. Like the myotomes of somites 

 four, five and six, it has disappeared phylogenetically, leaving 

 the eye muscles as an isolated group unconnected with the post- 

 otic myotomes (figs. 19 and 20). 



During the phylogenetic transformation of the eye muscles 

 noteworthy changes occur in their nerve connections. Since, 



Larval Aninhioxus. 



r 



17 



/•i a'^a^ j-'j" fit 



Larval Pefrcmijzon. 



sp. g.s 



18 



/3iyp. m. 



Figs. 17, 18 Pre-gnathostome stages in the history of the eye muscles. Dia- 

 grams based upon the myotomic relations in Amphioxus and Petromyzon embryos. 

 All of the myotomes persist in Petromyzon (Koltzoff) as in Amphioxus, but 

 they split into dorsal and ventral moieties in the case of the five anterior ones. 

 Abbreviations the same as in figures 14 to 16. 



THE JOURNAL OF MORPHOLOGY, VOL. 30, NO. 2 



