STRUCTURE AND DEVELOPMENT OF REISSNER’s FIBRE. 3 
number of individuals of different species, I should be able 
to place on a secure foundation results wliich are so g’reatly 
at variance with the “ optic reflex: theory ” of Sargent. 
As the result of this investigation, extending now over a 
period of nearly five years, I am convinced that both in its 
origin in the brain, and in its ending in tlie sinus termi- 
nal is, the condition of Reissner’s fibre is altogether different 
from the account given of it by Sargent, and I shall hope to 
succeed in demonstrating that this fibre is not a nerve-tract 
at all, and that consequently it cannot possibly have the 
function assigned to it by that author. 
The investigation has necessitated the preparation and 
examination of series of sections of the whole or parts of the 
central nervous system of between three and four hundred 
individuals, which have been selected from nearly seventy 
species. I have also endeavoured to ascertain the function 
of Reissner’s fibre by means of experiments upon living 
fishes performed at the Laboratory of the Marine Biological 
Association at Plymouth. 'The results of these experiments, 
a short account of which has recently been published (’12), 
are quite in accord with the suggestion made by Dendy (’09) 
that the fibre and associated sub-comtnissural organ form 
pai't of an apparatus for automatically regulating the flexure 
of the body. 
The altogether unexpected proportions to which the work 
has attained have rendered it advisable to publish it in several 
parts. This, the first part, will be confined mainly to an 
account of the conditions observed in the Cyclostomes. 
'I'he literature list appended does not profess to be com- 
plete. It includes only the works referred to in tliis^present 
])art, and the reader is referred to the work of Sargent (’04) 
for a more complete bibliograpliy of earlier writings which 
have a bearing, more or less direct, upon this subject. 
It is with pleasure that I acknowledge my indebtedness to 
Professor Dendy, to whom the inception of the work was due, 
not only for his invaluable advice and ci-iticism during the 
progress of the work and in the revision of the manuscript. 
