HISTORY OF THE EYE MUSCLES 451 



however, these have been discussed at considerable length in 

 earlier papers by the writer ('09, '12, '14), and since no facts are 

 presented which are irreconcilable with the conclusions stated 

 above, it seems unnecessary to do more than refer to them here. 

 While the first myotome retains throughout phylogenesis its 

 primitive nerve relations to the oculomotor nerve, the somatic 

 motor nerve of the second myotome (the trochlearis) acquires a 

 dorsal chiasma and retains connection only with the dorsal moiety 

 of the muscle. The ventral, portion, malting with the third 

 myotome, becomes innervated by the abducens nerve, the 

 somatic motor nerve of a more posterior metamere. The con- 

 clusion that such modified metameric nerve relations may have 

 occurred through a process of nerve substitution or piracy is in 

 harmony with what we now know of the method of nerve histo- 

 genesis (Harrison '11) and of the primary independence of nerve 

 and muscle (Parker '10). Consequently the modified metameric 

 nerve relations of the eye muscles present no serious objections 

 to the phylogenetic conclusions reached in this paper. 



Objection to the foregoing description of the phylogenesis of 

 the eye muscles may be raised on the ground of the uncertainty 

 that Amphioxus represents a form ancestral to vertebrates. 

 To some this will seem a serious objection. But that Amphioxus 

 embodies more completely than any other existing animal the 

 general characteristics of the chordate type from which the Ver- 

 tebrates have sprung, is an opinion held by the great majority 

 of vertebrate morphologists. Familiarity with embryonic and 

 larval stages of Amphioxus, Cyclostomes, and Elasmobranchs 

 greatly strengthens the conviction that this opinion is sound. 

 The evidence presented in this paper is in full accordance with 

 the belief. 



An attempt to carry the history of the eye muscles back into 

 pre-chordate stages leads eventually to the problem of the origin 

 of the segmental musculature — in other words to the problem 

 of the origin of the mesoderm. The logic of the previous dis- 

 cussion would lead to the conclusion that originally the eye 

 muscles were metameric diverticula of the invertebrate intes- 

 tine. Further than this we could scarcely proceed. 



