No. I.] THE PRIMITIVE VERTEBRATE BRAIN. 51 



{a) That the relation of the neuromeres to the origin of their 

 respective dorsal or sensory roots is fundamentally the same in 

 all three regions of the brain in which neuromeres give rise to 

 sensory roots. 



{b) That all the neuromeres of the brain, whether giving rise 

 to sensory roots or not, degenerate before the adult stage of the 

 animal is reached. 



It has also been shown (see Appendix). 



3. That in all probability the mid-brain originally consisted 

 of two neuromeres, and that the III. and IV. nerves were the 

 segmental nerves of these segments. 



4. That the number of primitive Encephalic segments was 

 probably ten (six in the hind-brain, two in the mid-brain, and 

 two in the primary fore-brain). 



5. That the neuromeres of the spinal cord, opposite the meso- 

 blastic somites, are "intersomitic" ; that is, the centre of each 

 neuromere is opposite the space between two somites, or vice 

 versa; hence it is seen that nine mesoblastic somites exactly 

 correspond to the nine spaces between ten neuromeres. 



It is now a well-known fact that the segmented mesoblast of 

 the trunk extends into the head region, and according to the 

 investigations of Van Wijhe it is there divided into nine meso- 

 blastic head segments, or " Myotomes," as he calls them, which 

 theoretically correspond to the nine spaces between the ten 

 Encephalomeres. 



Conclusions. 



I consider that the primitive vertebrate brain consisted of a 

 series of segments similiar to those found in the embryonic 

 spinal cord, and that the encephalomeres probably held the 

 same relation to the mesoblastic head segments as the myelo- 

 meres do to their respective mesomeres ; that is, they were inter- 

 somitic, the centre of each neuromere being opposite the space 

 between two somites and giving off a mixed nerve from the 

 apex. 



The region known as the Encephalon is the result of a great 

 differentiation and specialization of the anterior segments of this 

 primitive structure. That differentiation first began and has 

 been the greatest in the most anterior segments, which may 

 account for the greater size of the folds in this region than in 



