124 H. V. NEAL 



After a careful reinvestigation of the genesis of the posterior 

 rectus muscle the writer is able to affirm with positiveness that 

 Dohrn ('01, '04) is correct in claiming the persistence of the 

 rudimentary muscle which Miss Piatt ('91) thought was tran- 

 sient in the embryo. On the contrary, it persists and forms, as 

 Dohrn stated, the anterior portion of the posterior rectus muscle, 

 and is innervated by the abducens nerve. 



The problem presented by the fact of the distribution of the 

 abducens nerve to two myotomes — Van Wijhe's second and third 

 ■ — is more apparent than real, since, as has been shown by Bar- 

 deen ('98) for mammals and by Johnston ('08) for Cyclostomes, 

 a bimeric distribution is the rule for somatic motor nerves. 

 Therefore, the distribution of the abducens to two myotomes, 

 instead of presenting a difficulty, constitutes still further evidence 

 of its morphological similarity with spinal somatic motor nerves, 

 c. Is the abducens of Gnathostomes homologous with the most 

 anterior spinal nerves of Petromyzon? The cyclostomes have no 

 posterior rectus muscle and no abducens nerve (Johnston '05 a). 

 Assuming the primitive character of the cyclostomes, in this 

 feature the conclusion seems inevitable, therefore, that in the 

 course of phylogeny one or two of the anterior spinal nerves of 

 the Cyclostomes have been converted into the abducens of the 

 Gnathostomata. The apparent similarity between the ventral 

 ramus of the anterior spinal nerves of Petromyzon (n'v. 2, fig, 

 77) and the abducens gives sufificient grounds for raising the 

 question whether there may not be an homology — partial or 

 complete — between them. The position attained by the muscu- 

 lature {my. 4 V.I., fig. 77) innervated by the nerve n'v. 2 is so 

 close to the eye as to make possible, through slight variations 

 in the course of phylogeny, its attachment to the eye-ball. The 

 nidulus of origin of both nerves is post-otic and somewhat ex- 

 tended, while the distribution is pre-otic. But here the resem- 

 blance ceases. 



Against the exact homology of these nerves, it may be urged; 

 first, that the myotome innervated by the abducens is a pre- 

 otic one, while that innervated by the nerve n'v. 2 is a post-otic 

 myotome; second, that the nerve n'v. 2 is the nerve of the fourth 



