Houser, Zhe Neurones of a Selachian. 79 
oration of certain neurones, only. The attainment of the de- 
sired end is far from constant, however. After experience had 
shown the rule, care was always taken to apply this technique 
only to those animals which had been in an active condition, 
fresh from the open sea, if possible. But even with this pre- 
cautionary recognition of physiological conditions, so far as they 
could be readily determined, there apparently yet remained 
some unknown factor which caused a negative result in some 
instances where it was least expected. 
3. Lron Haematoxylin. 
This reagent was imagined by HEIDENHAIN (’92) for refined 
cytological work, but it truly has a place in neurological inves- 
tigation. It is a most excellent stain for defining the internal 
structure of the nerve-cell, and also for the tracing of nerve- 
fibres. For the latter purpose, iron haematoxylin has proven 
itself preferable in this research to the stain of WEIGERT, since 
it defines the axis-cylinder instead of the myelin, permitting 
fibres to be followed through their ramifications entirely to the 
terminal arborizations. 
Fixation of the tissue may be done with any good fluid. 
Where the tracing of axones, only, is desired, 10% formalde- 
hyde cannot be surpassed; but where the aim is purely cytolog- 
ical, either the chrome-oxalic mixture of GraF or the fluid of 
FLEMMING will give superior results. For work on axis-cylinders, 
celloidin sections were made 30 micra thick; and for the minute 
study of the nerve-cell, thin sections were cut by the paraffin 
method. 
The sections were brought from distilled water into the 
mordant of 4% iron alum for two hours. The excess mordant 
was then rinsed away with distilled water. Staining with 0.5% 
aqueous ‘haematoxylin required at least four hours for entirely 
satisfactory definition. Clean tap-water was used for washing 
out the uncombined stain, since this appears to fix the lake 
more firmly. The stain was differentiated with a 2% solution 
of iron alum, frequently renewed. This process was observed 
with the microscope, and when the desired effect had been ob- 
