HISTORY OF THE EYE MUSCLES 



443 



of lateral line sense organs. Possibly, as many morphologists 

 have thought, the lens and the otic capsule were at once time 

 members of the series of lateral line sense organs. The dorsal 

 and ventral moieties of the first myotome subdivide again, thus 

 forming the four muscles innervated by the oculomotor nerve. 

 The dorsal element of the second myotome forms the superior 

 oblique muscle, while the ventral unites with the third myotome 



Fig. 10 A camera drawing of a parasagittal section of an 8-day (Naples) 

 Petromyzon embryo, showing the mesodermic segmentation. The auditory 

 placode lies just posterior to the third somite. E.xcept for the absence of the an- 

 terior cavity, the mesodermic segmentation is comparable with that of Squalus 

 fig. 8). E7it, entoderm; Ot, auditory placode; Som. 1, Som. 2, Som. 3, Som. 4 

 Som. 5, So7n. 6, mesodermic somites 1 to 6; Sp. spiracular pouch; Spl. splanchnic 

 mesoderm; Tb.N. neural tube (forebrain). 



to form the external rectus muscle. While, in Petromyzon, the 

 following myotomes split above and below the ear and persist 

 in the adult, in elasmobranchs the myotomes of the fourth, fifth 

 and sixth disappear in ontogeny, leaving an hiatus between the 

 eye muscles and those of the trunk and bringing about their 

 characteristic isolation as a group of muscles in the adult (figs. 

 17 to 20). 



