Arthur B. Lamb 201 



external rectus opposed one another, the former pulling the back of 

 the eye forward; the latter, backwards. This stage is reached in on- 

 togeny at a length of 21-22 mm. (Figs. 4, 5, 6). The four muscles then 

 have the rectangular radial arrangement described above. They have 

 all reached approximately the same degree of differentiation, which is 

 far in advance of the three remaining eye muscles. 



These speculations, based solely on ontogenetic evidence, require con- 

 firmatory phylogenetic evidence derived from a study of forms lower 

 than Acanthias. 



EEFERENCE LETTERS COMMON TO ALL FIGURES. 



Altd., abducens nerve. 



AM., anlage of muscle adductor mandibulEe. 



A. v., auditory vesicle. 



BC. I., first branchial cleft. 



Cil. g., ciliary ganglion. 



C. m,, stalk connecting the premandibular somites of the two sides. 



U., Temporary muscle derived from the 'mandibular somite; in Fig. 9, 

 external rectus muscle. 



Ect., ectoderm of dorsal surface of head. 



E. Rec, externuis rectus muscle. 



F., temporary muscle derived from the mandibular somite. 



Qa. ff., Gasserian ganglion. 



/. Obi., inferior oblique muscle. 



Inf. Rec, inferior rectus muscle. 



Int. Rec, internal rectus muscle. 



MB., mesenchyme connecting dorsal and visceral parts of somite II. 



NC, notochord. 



OC, oculomotor nerve. 



Op. St., optic stalk. 



Op. Ves., optic vesicle. 



Op. v., ophthalmicus profundus branch of fifth nerve. 



O. a. -v., ophthalmicus superficialis branch of fifth nerve. 



0. S. VII., ophthalmicus superficialis branch of seventh nerve. 



Ra. v., anterior root of fifth nerve. 



Rp. v., posterior root of fifth nerve. 



R. VII., root of seventh nerve. 



S. A., anterior head somite. 



»SV., 811., 811 1., first (premandibular), second (mandibular), and third 

 (hyoid) somites. 



8. Ohl., superior oblique muscle. 



8. Rec, superior rectus muscle. 



811. D. d: v., dorsal and ventral portions of somite II. 



Tr., trochlearis nerve. 



YII., seventh nerve. 



All of the figures except 2 and 9 are drawn from vs^ax reconstructions. 

 The regions ruled on figures 4 and 7 are those portions of the somites which 

 are being transformed into muscles. 



