NEUROMEEES AND METAMERES 311 



The results of Locy ('95) and Hill ('00) form a very insecure 

 foundation for a belief in the metameric value of neuromeres. 

 For they are in disagreement with those of Eycleshymer ('95), 

 Neal ('96, '98, '14)', Kingsley ('97), von Kupffer ('06), Wilson 

 and Hill ('07), Griggs ('10), Graper ('13), Smith ('14). 



The neuromuscular relations of the rhombomeres described 

 in this paper and in a former paper ('14) are hard to reconcile 

 with the assumption of the metameric value of neuromeres. 

 Even with a strong prejudice in favor of the assumption, the 

 central nidular relations of the rhombomeres are such as to 

 throw much doubt upon the belief that neuromeres are trust- 

 worthy criteria of the metamerism of the head. 



No such doubt attaches to the- segmentation of the mesoderm 

 which manifests in the embryos of lower Chordates a series of 

 homologous segments concerning which observers are in very 

 general agreement. The mesodermic somites therefore afford 

 the most reliable criteria of the primitive metamerism of the 

 head. It is a wholly gratuitous assumption to assert that myo- 

 tomes have disappeared in phylogeny anterior to the first per- 

 manent myotome of lower Chordates. 



Consequently, until some evidence to the contrary is presented, 

 the writer feels compelled to retain the opinion expressed many 

 years ago, that the chief evidence of the metameric value of 

 neuromeres consists in their numerical correspondence with the 

 mesodermic somites. Such a correspondence, however, obtains 

 in the head region of vertebrates for only the primary brain 

 vesicles (neuromeres I to VII of the author) and not for the 

 secondary subdivisions of these (such as rhombomeres 1 and 2, 

 which result from the secondary subdivisions of neuromere III). 

 The larger size of the three anterior vesicles (I to III) presents no 

 difficulty to this view, since this difference may be attributed to 

 the increased functional importance of these brain divisions. 



Except in the case of 'neuromeres' II and III (Neal), however, 

 the motor nerve relations of the neuromeres do not accord with 

 the supposition that these are metameric structures. 



