306 H. V. NEAL 



roots of these nerves to the rhombomeres are alsb constant. The 

 central niduli of origin of these nerves are found to be the same 

 in mammals (Graper) and in the Elasmobranchs. Of the con- 

 stancy there seems to be no doubt whatever. But the con- 

 stancy has significance chiefly as evidence of the constancy of in- 

 dividual rhombomeres and not as proof of their metameric value. 

 The significant fact is that the constant nidular relationships of 

 the cranial motor nerves are not such as to justify the preva- 

 lent assumption of their metamerism. They certainly do not 

 correspond with the relationships assumed in many schemes of 

 cephalic metamerism, including those of the writer ('98 a, '98 b) 

 and Johnston ('05). 



In the light of the foregoing evidence, are we to accept neuro- 

 meres as trustworthy criteria of the metamerism of the verte- 

 brate head? If we do, we must not only shut out eyes to the 

 difficulties and objections just mentioned, but we shall also need 

 to answer the question. Whose neuromeres shall we accept as 

 criteria of metamerism? Shall they be those of Kupffer ('85) 

 or those of Locy ('95) and his pupil. Hill ('00), those of the 

 writer ('96, '98), or those of Griggs ('10)? Even if we disregard 

 the evidence of the non-metameric nerve relations described in 

 this paper and the lack of numerical correspondence of neuro- 

 meres and branchiomeres, there remains the unanswered question, 

 Which neuromeres are the Hrue' neuromeres? In the opinion 

 of the writer, this question may not be answered regardless of 

 the metamerism of the mesoderm, as has been so often done by 

 students of neuromerism. The metamerism of the mesoderm is 

 primary while that of the nervous system is secondary. There 

 must therefore have been at one time a correspondence between 

 the neuromeric and mesomeric segmentation of the head. Such 

 a correspondence obtains for the primarj^ brain divisions of 

 Squalus (neuromeres I to VII of the writer) and the mesodermic 

 somites. In this numerical correspondence may be seen the 

 most convincing evidence of the metameric value of neuromeres. 



Of the metamerism of the mesoderm there is no resonable 

 doubt in the light of the very general agreement among mor- 

 phologists concerning the mesodermic somites of the head. The 



