THE MICROTOMWIST'S VADE-MECUM

necessary in order to obtain a thorough stain, maceration of
the tissues supervening. But alcoholic staining fluids have
still other advantages; they are vastly more penetrating;
with them alone is it possible to stain through chitinous in-
teguments; they stain more quickly; the stain as a rule is
more precise; and, if it be desired to stain slowly, tissues may
be left in them for days without hurt.
There are a few exceptional cases in which aqueous staining
media are admissible, and some in which they are preferable
to alcoholic media. Thus Mayer finds that picro-carmine is
less hurtful to tissues than other aqueous stains, and attri-
butes the result to the picrin contained in the solution; and       heek
other workers have found that alum-carmine gives very good
results when employed after alcohol. And in general with
tissues that have been well hardened in chromic acid the            tre~at
action of aqueous fluids does not appear to be seriously hurt-      Si hi
ful. Mayer observes that with certain groups of animals it          I doD
is necessary to allow the tissues to be reimbibed and swelled       of th
up with a watery fluid in order to obtain the required action       ptol
of the colouring agent. Lang finds that Turbellaria can only
be successfully stained by means of an aqueous solution of
picro-carmine and eosin.                                             Ih fI
Te
pieIh
iiii
Of'__  ____~                                   ____ -%W_

10


K