106 



are givea off. Ths diamet-ar of the process remains unifor'n 

 throut^hoat except for slight swelliai^s v?hich occur at inter- 

 vals. The fibre terminates at the periphery of the brain. 



The fact that the ependynal fibre becomes irregular and 

 branches only after it leaves the central gray matter for the 

 ne-ver additions outside may have a phylogenetic significance, 

 indicating that at one time the process had no farther course 

 than the outer limit of this most ancient nerve-substance. 

 But it is also entirely possible that the phenomena in ques- 

 tion are /without such deep significance, having been caused 

 by the greater number of obstacles in the path of the fibre 

 as it dre^ through the outer levels. 



4. Phylogeny of Miibrain Structures. 



The midbrain has ever been a stable part of the neural 

 tube. Marked out early in ontogeny from the other brain-seg- 

 ments, the midbrain of Mustelus retains many features of organ- 

 ization which are really primitive in character. 



The central gray matter is the most archaic of the mid- 

 brain structures, and the newer additions of the outer levels 

 are derivable from it. The central gray matter is to be com- 

 pared, both as to general functions and morphological topog- 

 raphy, to the gray matter of the spinal cori before the latter 

 has developed its specialized cornua. In a broad way, the 

 ventral region of each is motor, and the dorsal part a series 



