NEUROMERES AND METAMERES 303 



The peculiar relationships of the abducens nerve have been 

 discussed in an earlier paper (Neal, '14, pp. 118-122), and have 

 been found reconcilable with the hypothesis of the primar}^ 

 metamerism of the neuromeres (rhombomeres) . The fact that 

 Graper ('13) finds the nidular relations of the cranial nerves of 

 mammals identical with those described in this paper for Elas- 

 mobranchs would seem to exclude the likelihood of a phyloge- 

 netic migration of the motor niduli. The ontogenetic shifting 

 of the relative position of the visceromotor and somatic-motor 

 niduli occurs in Squalus precisely as in mammals (Graper, '13). 

 This is shown in figures 9 to 13 of this paper. But this migration 

 involves no metameric shifting of the motor niduli from one 

 neuromere to another. The nerve relations described above, 

 moreover, may not be explained as the result of a metameric 

 shifting of muscles associated with the cranial nerves, since of 

 such migration there is neither comparative anatomical nor 

 comparative embryological evidence. Consequently, upon the 

 assumption of an original metamerism of the rhombomeres and 

 of their associated muscles, it is necessary to assume that the 

 motor cranial nerves have invaded territory once foreign to them. 



It is not easy, however, to discover the conditions which would 

 lead to such a series of nerve substitutions. Comparative em- 

 brj^ology and anatomy throw little light upon the problem. The 

 problem presented by the relations of the abducens has already 

 been discussed at length by the writer ('14). The nidular rela- 

 tions of the facialis nerve (figs. 1 and 2) are equally hard to in- 

 terpret. Assuming the original metamerism of the rhombomeres, 

 why should motor fibers of four (4 to 7) rhombomeres be dis- 

 tributed to a single visceral arch? In Amphioxus, the fibers of 

 four metameric nerves are distributed to the velar musculature. 

 How this plexus arose is uncertain. Its appearance ma}- be 

 correlated with the enormous backward extension and later 

 recession of the mouth in larval Amphioxus. If a similar back- 

 ward extension of the mouth occurred in the ancestors of Verte- 

 brates and a similar plexus of four metameric nerves were 

 formed, the results would resemble in all essentials the rela- 

 tions shown by the facialis. Ontogenetic support for such a 

 supposition, however, is wholly lacking. 



