MORPHOLOGY OF EYE MUSCLE NERVES 137 



Therefore, in view of the disagreement among students of 

 neuromerism regarding the nature sind number of true neuro- 

 meres, and the persistent doubt regarding the results of Locy 

 and Hill, morphologists may still feel skeptical regarding any 

 scheme of metamerism based upon the segmentation of the 

 nervous system and uncontrolled by the evidence of mesodermic 

 segmentation. 



The writer finds himself in agreement with Belogolowy ('10 a, 

 p. 510) in the opinion that the neuromeres have only a secondary 

 importance as criteria of the primitive segmentation of the head. 

 The latter states (p. 515) that: 



Without ha\Tiig any organic relations to the functional activities 

 of the nervous system, and presenting merely form changes of the 

 neural tube, the neuromeres in my opinion, can serve at best merely 

 as topographic landmarks in the stud}^ of the nervous centers. The 

 complete lack of any satisfactory explanation of their appearance 

 and the indications of the possibility of their purely secondar}^ formation 

 under the influence of this or that mechanical factor acting on the 

 nervous system — ^as in the case of the constrictions of the spinal cord 

 under the pressure of the somites — •limits to the utmost their employ- 

 ment as criteria of metamerism. 



Johnston's assertion ('05, p. 234) that "nervous structures 

 represent more segments than have preserved in the mesoderm;" 

 that ''in other words, there are preserved vestiges of nerve struc- 

 tures belonging to segments whose entodermal and mesodermal 

 organs have disappeared for the most part" begs the entire ques- 

 tion. The fact that the brain shows a larger number of divi- 

 sions — whether the problematical marginal headings of Locy or 

 the secondary subdivisions of the differentiated tube — than does 

 the mesoderm, does not prove that the nervous divisions, are 

 ancestral or primitive. Moreover, there is no reason for assum- 

 ing that mesodermic segments have disappeared in the head 

 region of Squalus. As a matter of fact, the number of somites 

 in Squalus corresponds with the number of primary brain ves- 

 icles. In this numerical correspondence we have the strongest 

 proof of the metameric value of these two segmental structures. 

 Moreover, this inference accords more fully with the conclusions 



