FIXING AGENTS

results in many cases. Bichromate of potash is a very useful
reagent of general applicability.
The following are the points that it is most important to
bear in mind:
Osmic acid must not be used with impermeable tissues.
Kleinenberg's solution is less adapted for the tissues of
vertebrates than for those of invertebrates, on account of its
causing connective tissue to swell.
0j ~  Chromic acid is a very safe reagent, if not used in
ator  too strong solutions, and may always be used in case of
doubt.
4ad  In the case of marine organisms it may be stated as a
tiSu  general rule that their tissues are more refractory to the
t  action of reagents than are the tissues of corresponding
fresh-water or terrestrial forms, and fixing solutions should
in consequence be stronger (about two to three times stronger,
according to Langerhans).
6. Heat as a Fixing Agent.-In some cases the simple
application of heat is not only a good means of fixing, but the
very best. Ova of some Arachnida, of Pycnogonida, ova and
larva of some Bryozoa, are better fixed by this means than by
errtd  any other. It will probably be found that this method is
On    very helpful in the case of minute structures enclosed in very
Utit  impervious cases of chitin or other impervious material;
such cases or capsules obviously opposing but an inappreciable
r, ad  resistance to the passage of heat waves. The method consists
from  simply in heating the objects to the temperature necessary to
kill the cells and coagulate their contents. This is done by
heating them   in water in a watch-glass. It is, of course,
l io S better if practicable to throw them at once into water
, an  previously heated to the required temperature. I am       not
geof  able to state precisely what that temperature should be; it
tati5  appears, however, that a few seconds' immersion in boiling
) and  water is not hurtful even to delicate tissues.
siRbe    The method has the great advantage of allowing of good
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