140 GEORGE E. NICHOLLS 
In all, experiments were performed upon sixty-seven elasmo- 
branchs, of which twelve were dogfish and the remaining fifty- 
five were rays. Two only, as already mentioned, died from the 
effect of the anaesthetic, while two others died from suffocation 
consequent upon my omission to replace the aerating tube in the 
tank after the specimens had been photographed. 
They were killed by being plunged into a mixture of spirit 
and chloroform and, after a brief stay in this fluid, were evis- 
cerated. In this way the blood vessels were practically drained, 
which greatly facilitated the rapid dissection necessary to expose 
brein and spinal cord, there being no troublesome effusion of 
blood from cut vessels within the brain case. The partially 
dissected specimens were immersed in a large vessel of fixing 
fluid (Tellyesnicky’s bichromate-acetic mixture) and the further 
dissection required to expose the greater part of the spinal cord 
wes completed under the fluid. To dissect away the vertebral 
column from the hinder part of the spinal cord and the filum 
terminale is, however, a very delicate operation, which involves 
considerable risk of damaging the nervous system. The exposure 
of the spinal cord was, therefore, carried only to within a couple 
of inches of the end of the tail. Behind this point I was con- 
tent to strip away most of the skin and muscles, about half an 
inch at the actual extremity being left quite untouched. In the 
case of the dogfish the last inch (or even more) of the tail was 
left intact. 
The preparation of the series of sections proved unexpectedly 
difficult. In general, a piece of the tail, about an inch in length, 
was removed—this piece including the point of experimental 
lesion—and prepared for sectioning. 
My intention was to cut this terminal piece sagittally in order 
that the point of experimental incision and a_ considerable 
length of the filum terminale before and behind this point 
might be seen in one and the same section. To avoid risk of 
damage to the sinus terminalis it was found expedient to retain 
undisturbed the skin upon the last half inch or so of the tail and 
the terminal piece, therefore, contained the bases of numerous 
spines embedded in the skin, and separated from the axis of 
