244 FISH. [Vol. X. 



forms where it is excessively developed, is apparently an excep- 

 tion to the tripartite rule. 



In the Amphibia and in the early stages of some other forms 

 the cerebellum is nothing more than a commissure and is an 

 azygous or mesal part as is the epiphysis and infundibulum, 

 while in the Hag fishes it is quite likely there is no cerebellum 

 at all. There is moreover below mammals no pons to help us 

 over the difficulty of finding a dividing line between the epen- 

 cephal and metencephal or differentiating a pre- from a post- 

 oblongata. The infundibulum and the parts connected with it 

 in many animals show this triple arrangement which, with its 

 probable intimate relation to the morphological front of the 

 head in the embryo, has led some to believe that this region 

 might also claim the distinction of a separate segment. Kupffer 

 (24) has proposed the term hypencephalon ; the same author, 

 contrary to common usage and all precedent, applied the term 

 epencephalon to the great brain or prosencephal. 



Physiologically there is more or less of a dual connection of 

 the brain throughout all of these segments and from the stand- 

 point of a duplicity or bilaterality of the parts there are mor- 

 phological as well as physiological grounds for admitting the 

 rhinencephal as a segment. 



In the lancelet the olfactory is the most strongly differ- 

 entiated portion of the neuraxis ; but the asymmetry and the 

 absence of either duplicity or triplicity of this region render 

 this animal an exception to the general vertebrate rule. Mrs. 

 Gage (13) finds that in the embryo of Diemyctylus "there are 

 two portions of the forebrain — one associated with the de- 

 veloping olfactory nerves, the other lying next the diencephal 

 — with a large common cavity." 



The cephalic part of the cavity belongs to the olfactory 

 region and the caudal to the cerebrum, a condition approxi- 

 mately retained in the lamprey, and she therefore concludes 

 that the rhinencephal is quite legitimately entitled to a share 

 of the aula as a mesal cavity, and consequently possessing a 

 tripartite arrangement. 



Steiner (52) after experiments upon the different segments 

 of the brain of some of the lower vertebrates, especially the 



