THE MICROTOMIST'S VADE-MECUM

fibre in all of its physiological states may be obtained and
preserved as records of moments whose duration is measured         r late
by fractions of a second.                                         p5A
The processes that have been imagined for the preparation       cry
and permanent preservation of protozoa form another case in
point. By simply fixing the animals on the slide by means           4
of picric acid or osmium   they may be stained, cleared, and       k
mounted either in an aqueous medium, or in balsam, as
successfully as the tougher tissues of higher organisms. To
attempt to mount them     in any medium without first sub-        5.
jecting them to that preliminary hardening brought about by        It
the use of fixing agents would be hopeless. Lastly, I may
give an example of the usefulness of fixation in cases in which     IRC S1
it is desired to prepare contractile organisms in their natural    Ca sS, S
extended state. Suppose you wish to mount a hydra with             staq
the tentacles extended, it is obvious that the animal must be      o
killed with the utmost rapidity or the tentacles will be with-     of soln
drawn before death, and you will have no means of getting          eeritis
them extended afterwards. If the animal be placed in a cell         telst
with a few drops of water, and if as soon as the tentacles are
fully extended you add one drop of 2 per cent. solution of         tihatW
osinic acid the hydra will be killed with such lightning speed     igh
that not a tentacle will be retracted (Ranvier). bae-
In blood we have an example of a tissue of which it is
simply impossible to obtain permanent preparations well
preserved without previous fixing. But if blood be treated,
as a first step in the process of preparation, with picric acid
or corrosive sublimate, it may be stained and mounted in
some preservative medium with such success as to be avail-
able for the most minute study. And the same is true in
general of other tissues. " Fresh tissue-elements or portions
of tissue cannot be kept as preparations without undergoing
modifications, such that after a few days their whole character
is changed. Certain liquids have for a long time been recom-          01
mended as having the property of preserving indefinitely
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