114 H. V. NEAL 



fully developed, extended dorsally to form a union or interdigi- 

 tation with the antmieric myotome. As Flirbringer has correctly 

 stated, no skeletal structures would prevent the extension of 

 fibers across the median plane. It may be imagined that this 

 extension of muscle fibers across the median plane was corre- 

 lated with the muscular development of the prostomial region. 

 Petromyzon still shows (fig. 77) the extension of myotomes into 

 this region. Under such conditions shght variations in the length 

 of the nerve fibers which grow to connect with these muscles 

 might bring about a peripheral chiasma. The possibility that 

 such muscles had connections with the epiphysis is not excluded, 

 but such a supposition does not seem necessary. Changes in 

 the extension and direction of growth of muscle and nerve fibers 

 in this region may have been correlated with the development 

 of the cephahc flexure which would seem to require some adjust- 

 ment of the muscles since the forebrain and midbrain regions 

 were flexed into a more \;entral position. The final result of the 

 flexure, however, appears to have been a shifting of those por- 

 tions of the musculature which persisted in this region into a 

 more ventral position and a separation of the muscles which 

 had been apposed in the median plane above the brain wall. 



The growth and great enlargement of the lateral eyes also 

 brought about changes in the (Van Wijhe's) second myotome, 

 which became split into dorsal and ventral moities {my. 2 v.L, 

 my. 2 m., 7ny. 2 d.l.) in precisely the same way as occurs onto- 

 genetically in the post-otic ' muscles of Petromyzon as a result 

 of the growth of the otic vesicle (figs. 78, 79). 



It may be assumed that, as in the latter case, the median 

 portion of the myotome degenerated, together with its branch 

 of the somatic motor root (trochlear nerve), while the lateral 

 moiety became innervated by a branch of the abducens (fig. 

 82, abd.). The dorsal moiety, however, retained its connection 

 with the trochlear nerve, and possibly also with fibers from both 

 sides of the brain, by means of a dorsal, peripheral chiasma. 

 Then, when later this dorsal moiety degenerated with the excep- 

 tion of that portion which became attached to the eye-ball to 

 form the external oblique muscle (crossed hatched in the dia- 



