EVAPORATION IMBEDDING-MASSES

"q  object should have been taken) has so far shrunk that the
object begins to lie dry, a drop of collodion is added, and the
whole left as before. This process is repeated every few
hours for two or three days, at the end of which time it
will generally be found that the object is imbedded in a
magma of half-dry collodion of sufficient hardness for see-
tion-cutting. The mass is then scooped out of the mould
and preserved till wanted in 70 per cent. or 80 per cent. alcohol.
(Soft tissues do not shrink under this treatment. I have
obtained by this means admirable sections of the most deli-
cate sponges, for instance.)  I cut with a knife wetted with
. 70 per cent. alcohol, and float my sections into a watch-glass
with 70 per cent. alcohol, which is gradually changed for
.s  stronger up to 90 per cent. (Stronger must not be used.)
tht I then clear as rapidly as possible with oil of cloves or
a s  carbolic acid, and mount in balsam.    If the dehydration
l  with 90 per cent. alcohol and the clearing with clove oil be
dOSSor performed rapidly, the mass will not dissolve out; it softens
-DA1 and becomes perfectly invisible, and I believe it is this which
fOr L- may have led IDuval to believe that it was dissolved, whilst
reslIts in reality its presence was only masked by the identity of its
1ool refractive index with that of the clearing medium.
Win    Objects should be stained en masse before imbedding
ldi  unless the sections are to be fixed and stained on the slide.
feos  I consider that there is only one defect in this method, and
114  that is the impossibility of obtaining a mass hard enough to
admit of the cutting of the very thinnest sections, without
-la- causing shrinkage in the specimen. For the point at which
i Th the drying-up must be arrested in order that the tissues may
not suffer, is one at which the magma of collodion is not yet
objcts sufficiently hard from the mechanical point of view of section-
oceli  cutting. This defect in part is eliminated by the use of a
:ehe special collodion, viz. Schering's " celloidin " (see below). I
18    should mention that I was led to the method of partial and
Coer the gradual drying-up, by finding that when I followed Duval's

195