136 GEORGE E. NICHOLLS 
In the latter event the tangled end (or ends) might perhaps 
afford a temporary hold and so prevent the fiber from being 
put completely out of action. 
And, finally, there was the problem of regeneration. It was 
uncertain whether a tangle, if it were formed, would remain as a 
permanent record of the breaking of the fiber, or, if it were a 
transient feature, whether it would simply uncoil or whether the 
whole fiber, or the tangled part of it, simply disappeared to be 
replaced by a new growth. 
Upon some of these points a certain amount of light was shed 
by the results of the few preliminary experiments carried out in 
1910 but upon others the information was too meager to supply 
a decisive answer. Upon many of these points much additional 
knowledge has been gained from the more extended investi- 
gation carried out in the following year. 
Ill. MATERIAL AND METHODS 
The curiously exposed condition of the filum terminale in 
fishes, coupled with the fact that in both elasmobranchs and 
teleosts, Reissner’s fiber is particularly well developed, largely 
influenced my choice of material. My final preference for elas- 
mobranchs was determined by the idea that the absence of 
bony tissue in the vertebral column would facilitate the prep- 
aration of the inevitable large number of series of sections. 
I planned, originally, to experiment principally upon the 
common dogfish (Seyllium canicula) and to use rays only in the 
event of dogfish of suitable size being unobtainable. Knowing 
nothing certainly as to the probable extent of the recoil of the 
fiber, I was anxious to make use of comparatively small speci- 
mens, for it was possible that serial sections of the entire length 
of the central nervous system of all of the specimens might 
have to be prepared—a task of no little magnitude. 
As it happened, only a couple of reasonably small dogfish were 
obtained during my stay in Plymouth in July, 1910; relatively 
small rays were, however, moderately plentiful, and for the most 
part the preliminary experiments were performed upon these 
animals. 
