THE SEGMENTATION OF THE PRIMITIVE VER- 

 TEBRATE BRAIN. 



By CHARLES F. W. McCLURE, B.A., 



E. M. Fellow in Biology at Princeton. 



Part I. 



The primitive segmentation of the vertebrate brain is a 

 problem which has probably attracted as much of the attention 

 of morphologists as any one of the great, unsettled questions of 

 the day, and many views have been advanced which have, it is 

 true, reached one important point of agreement ; namely, that 

 the primitive brain was undoubtedly a segmented structure. 

 But beyond this, in regard to the character of these segments 

 and the number of segments of which the brain originally con- 

 sisted, I think it can be said with perfect freedom that nothing 

 whatever has been definitely proved. It is the purpose of this 

 paper to add a few more links to the chain of evidence neces- 

 sary for the elucidation of this important question. 



The majority of investigators on this subject have made use 

 of the cranial nerves as a means of determining the number of 

 segments of the primitive brain. Investigations in this line are 

 good as far as they go, but as far as the determination of the 

 original number of segments and the character of these seg- 

 ments by this method is concerned, it is largely conjectural for 

 the following reasons : 



I. We have positive proof that the degeneration of certain 

 branches has taken place.^ This being the case, we have every 

 reason to assume that whole segmental nerves may have once 

 existed, which have completely degenerated, leaving no trace 

 whatever of their previous existence. If such be the case, the 

 segments originally connected with these degenerated nerves 

 must necessarily be overlooked, if the existing nerves are made 



1 Marshall states that the IV nerve possesses a sensory branch in Selachians and 

 Amphibians. Gegenbaur notes the same for Selachians. 



