MORPHOLOGY OF EYE MUSCLE NERVES 125 



and fifth post-otic myotome while the fourth and fifth post-otic 

 myotomes of Squalus (Van Wijhe's 7th and 8th) have their own 

 somatic motor nerves, namely, the anterior roots of the embry- 

 onic hypoglossus (fig. 81), which must therefore be the exact 

 homologues of the nerve n'v. 2. 



The relations are such, however as to indicate that the abdu- 

 cens is the intrinsic nerve of Van Wijhe's 4th and 5th somites. 

 The transient ramus recurrens of the abducens (VI, rec, fig. 76) 

 may possibly indicate the former relations of the abducens to 

 the 6th somite or at least to posterior myotomes. If this con- 

 clusion be correct, then it follows that Petromyzon has, in the 

 course of phylogeny, lost the homologue of the abducens with 

 the loss of the median portions of the post-otic myotomes. 



It will be recalled that the abducens has a ramus recurrens, 

 observed by Dohrn ('90, '01), Miss Piatt ('91), Neal ('98) and 

 Belogolowy ('10), and interpreted by them as evidence of a 

 former posterior distribution of the abducens. If this conclu- 

 sion be correct, the homology of the abducens with the lost 

 nerves of Petromyzon is rendered more certain. 



One of Belogolowy 's ('10 a) most important discoveries is the 

 fact that in chick embryos the roots of the abducens and of the 

 hypoglossus form members of a continuous series of anastomos- 

 ing roots. By such evidence their serial homology seems clearly 

 demonstrated. Moreover, he also finds roots of the abducens 

 arising from the second hindbrain neuromere — a pre-otic neuro- 

 mere — and infers that this root is a remnant of the primitive 

 nerve belonging to the third somite. Belogolowy lays consid- 

 erable stress upon the fact that anastomoses are formed between 

 the abducens and the oculomotor fibers, resulting in relations 

 resembling those between the abducens and the hypoglossus. 

 Altogether the evidence seems overwhelming in favor of the 

 view that all of these nerves form members of a continuous 

 series of homologous nerves, and that the assumed distinction 

 between post-otic and pre-otic regions of the head is arbitrary 

 and artificial. 



d. Conclusions regarding the morphology of the abducens. That 

 the abducens is a somatic motor nerve innervating somitic mus- 



