THEORY OF STAINING

a plausible exception, all alkaline staining solutions should
be avoided.    Alkalies dissolve nuclein, or if they do not
dissolve it when very dilute, swell and distort nuclear struc-
tures, and are frequently hurtful to formed material. Neutral
or acid stains should alone be used, and it will probably be
found that better preparations are obtained with acid solu-
tions than with neutral ones. It is most important to work
with acid stains in all cases in which it is desired to faith-
fully preserve nuclein formations (Carnoy, 1. c., p. 211).
In order to obtain precise stains it is important to operate
on tissues that have been carefully fixed; or, if fresh tissues
'od.  be taken, that the staining solution should itself be a sufficient
fixative. An important progress has been lately realized by
the discovery that it is possible to combine the fixative action
of osmic acid with some stains (see Alum-carmine and Osmium
and Methyl-green, Carnoy's method) by adding a trace
of the acid to the staining solution. It is to be hoped that
the means of combining 'osinic acid with many other stains
WT    will be discovered, as the fixing action of osmic acid would be
el is  most desirable in cases in which staining is performed by
OtioS means of prolonged immersion in an aqueous medium. Carnoy
states that he employs osmium     with almost all his liquids
10   (picro-carmine is an exception, as osmic acid precipitates it).
iu   In the same way, acetic acid (which is a good fixative for
:tih nuclei) may be combined with many stains; for instance, with
u@  alum-carmine, methyl-green, safranin, gentian violet, Bis-
was    marck-brown, nigrosin, and others (Carnoy).
Et of    In the Introduction I have pointed out the desirability of
p  lio  employing alcoholic stains for objects that have been treated
MeS    with alcohol; it will be sufficient here to again call attention
urely  to the superior penetrating power of alcoholic solutions. The
in to  histologist should never be without a good alcoholic stain.
The question of permanence of stains has some importance.
Carmine is certainly, in my opinion, the colouring matter that
can most be relied on in this respect. It is affirmed by a

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