118 H. V. NEAL 



The second and more important point of difference between 

 the abducens and spinal somatic motor nerves is its distribution 

 as a post-otic nerve to pre-otic myotomes. Instead of inner- 

 vating musculature of its own metamere, it is distributed to 

 myotomes of anterior metameres (fig. 76). This relation, there- 

 fore, demands interpretation. 



a. How may the innervation of pre-otic musculature, the pos- 

 terior rectus muscle, by a post-otic nerve, the abducens, be inter- 

 preted? Are the myotomes innervated by the eye-muscle nerves post- 

 otic myotomes which have migrated into pre-otic territory? In the 

 attempt to solve the problem presented by the distribution of 

 the abducens, namely, the problem of the distribution of a post- 

 otic nerve with a post-otic nidulus to pre-otic muscle, two alter- 

 native hypotheses suggest themselves. 



According to the first hypothesis, the posterior rectus muscle 

 is to be regarded as a post-otic muscle which has migrated in 

 the course of phylogeny into pre-otic territory, carrying with 

 it the associated nerve, the abducens. As would be expected 

 the nidulus of the abducens has retained its primitive position 

 in the medulla, posterior to the otic capsule. One of the con- 

 ditions which has brought about the migration of the posterior 

 rectus muscle may have been the reduction and final disap- 

 pearance of the pre-otic nmscles, through the development and 

 hypertrophy of the sense organs, cranial ganglia and carti- 

 lage cranium. Then, after the atrophy of the pre-otic mus- 

 cles, post-otic myotomes invaded the territory in the same way 

 as occurs ontogenetically in the case of the anterior trunk 

 somites of Petromyzon (figs. 77 and 78). This evidence from 

 Petromyzon meets an objection which may be raised against the 

 theory, namely, that it is unreasonable to suppose that if the 

 pre-otic region became too crowded to retain its own muscles it 

 would be. able to contain muscles from elsewhere, since this is 

 precisely what seems to occur ontogenetically in this animal. 

 In Petromyzon the pre-otic somites break up into loose mesen- 

 chyma and in later stages post-otic myotomes invade the terri- 

 tory. A comparison of figure 77 with figure 81, shows how 

 similar the relations of the associated nerve in Petromyzon {nv. 

 2, fig. 77) and of the abducens nerve of Squalus are. 



