638 JOURNAL OF COMPARATIVE NEUROLOGY. 
pendent upon methods of investigation that the conceptions 
held by us might almost be said to grow out of the technique 
employed. The development of neurological methods during 
the last few years has been indeed phenomenal, and with this 
advance there has come the necessity for a re-investigation of 
many nervous systems. The results obtained by the earlier 
observers, while praiseworthy in themselves, simply do not fur- 
nish the precise and complete pictures of neurones which mod- 
ern comparative neurology requires. And hence it is that the 
writer has attacked anew the structural problems ofa brain which 
has by no means escaped the attention of investigators. The 
phylogenetic value usually assigned to the Selachii has caused | 
many to examine the brain of the shark, both anatomically and 
microscopically. A historical review of the latter class of re- 
searches will be found in Section II, 2. 
The results set forth in this paper are intended to further 
exact knowledge concerning the external morphology, the in- 
ternal organization, and the architectural relations of the sela- 
chian neurones, while also aiding, it is hoped, in the elucidation 
of certain questions of a general character. The particular 
selachian selected, J/ustelus canis of DEKay, is one in which 
are combined a structure fairly representative of the Selachii as 
a whole, a size convenient for work, and availability in sufficient 
numbers to meet the rigorous demands of certain methods of 
investigation. 
This research was begun at the Marine Biological Labora- 
tory of Woods Holl, where two seasons were spent in the use 
of an abundance of living material. The principal study was 
continued at the University of Iowa as my regular duties per- 
mitted ; while certain collateral lines were followed in the neu- 
rological laboratory of the University of Chicago. A short pre- 
liminary notice of the most important results then in hand was 
published in the Proceedings of the lowa Academy of Sciences, 
Volume IV. The research has finally been brought to com- 
pletion in the biological laboratory of the Johns Hopkins 
University. 
It is a real pleasure to acknowledge my indebtedness to 
