MORPHOLOGY OF EYE MUSCLE NERVES 123 



posterior rectus muscle in the chick consists of two or three 

 masses, but he does not suggest that this division indicates a 

 metamerism of the muscle. 



Upon the basis of observations upon Torpedo embryos Dohrn 

 ('90) inferred that each of the eye-muscle nerves has a polymeric 

 origin and distribution, the abducens representing three or four 

 metameres, the oculomotorius possibly a larger number, and the 

 trochlear a single metamere. A year later, however, he con- 

 cluded that the m. obliquus superior is developed from two myo- 

 tomes. JRegarding the number of somites in Torpedo, however, 

 Killian ('91) and Sewertzoff ('99) reached conclusions divergent 

 from those of Dohrn, and Dohrn ('91, '01, '07) has repeatedly 

 revised his conclusions, and in his latest paper ('07) decides that 

 the second somite of Van Wijhe represents three or four myo- 

 meres. In this connection the statement of Sewertzoff ('98) that 

 the somites of Dohrn ('90) and Killian ('91) in Torpedo second- 

 arily merge into those of Van Wijhe and that the myomeric 

 segmentation of Torpedo and Pristiurus is identical, is important. 



Miss Piatt ('91) had noticed that just anterior to the anlage 

 of the posterior rectus muscle there appears the rudiment of a 

 large muscle derived from the posterior portion of the second 

 (mandibular) somite. This muscle, according to Miss Piatt, soon 

 degenerates and her conclusion has been confirmed by Lamb 

 ('02). Johnson ('13) thinks that he is able to identify the same 

 rudiment in Chelydra embryos. But Dohrn ('01, '04) denies 

 its degeneration and affirms its persistence as an integral part 

 of the definitive posterior rectus muscle, the major portion of 

 which is derived from the third somite of Van Wijhe. 



Sewertzoff ('99) derives the posterior rectus muscle of the 

 selachii from Van Wijhe's third somite, which he regards as the 

 first true somite, and which he states is formed by the conflu- 

 ence of two primary somites. Sewertzoff makes the interesting 

 discovery in Torpedo that the third and fourth somites unite to 

 form the posterior rectus muscle. This divergence of opinion 

 regarding the metameric relationships of the posterior rectus 

 muscle is important and the whole problem should be reinvesti- 

 gated in embryos of both Squalidae and Torpedinidae. 



