MORPHOLOGY OP* EYE MUSCLE NERVES 121 



ure may perhaps be ascribed to the small size of lamprey em- 

 bryos and the great difficulty of identification of embryonic 

 nerves. The differentiation of muscle fibers, however, as Harri- 

 son has shown, does not imply the presence of nerves. 



The portions of these myotomes lying lateral to the ear per- 

 sist into the adult and form the anterior segments of the lateral 

 trunk musculature (fig. 77; my. 4 d.l., my. 4 v.l.). They are 

 innervated by branches of nerves of posterior myotomes, namely, 

 those of the fourth and fifth post-otic myotomes. That each 

 of the anterior myotomes was at one time innervated by its own 

 segmental somatic motor nerve seems indisputable, and some 

 explanation of the present modified relationships seems required. 

 We may assume that the nerve {rx.v.) {;n'v. 1 and n'l;. 2, fig. 77) 

 which innervates the five most anterior post-otic myotomes con- 

 sists of the combined nerves of these five segments, each of 

 which retains its primary connection with its related myotome. 

 This assumption harmonizes with the hypothesis of 'the primary 

 continuity of nerve and muscle. Were this assumption correct, 

 however, we should expect to find the nidulus of the nerve ex- 

 tending anteriorly as far as the otic capsule. Of this there is 

 no evidence. The only neuroblasts, the processes of which can 

 be traced into the roots of this nerve, lie solely in the region of 

 the roots. 



As an alternative explanation of the nerve and muscle rela- 

 tions under discussion, it may be assumed that the nerves asso- 

 ciated with the three anterior myotomes have degenerated, while 

 their area of distribution has been usurped by the nerves of 

 posterior myotomes. The degeneration of the median division 

 of the first post-otic myotome may have been one of the con- 

 ditions which lead to this substitution in the case of this myo- 

 tome. Such an assumption of nerve substitution is in harmony 

 with the rapidly increasing evidence in favor of the process 

 theory of nerve development and with the limitation of the 

 nidulus of the nerve as stated above. 



Furthermore, the evidence presented by Johnston ('08) and 

 his conclusions, support this assumption. In his discussion of 

 the segmental relations of ventral nerves in Petromyzonts John- 



