THE MICROTOMIST S VADE-MECUM

for which absolute alcohol is at last substituted. After a few
days the paper may be removed and the mass cut.
It is advantageous to clear the mass before cutting by
soaking it for a day or two in clove oil or turpentine, not-
withstanding that this process makes the mass somewhat
softer, the perfect transparency imparted to the mass being
advantageous for placing the object in position. The sections
obtained may be mounted direct in balsam without freeing
them from the mass, which under the microscope is almost or
quite homogeneous.
If the mass should have become overhard in the alcohol,
it may be softened by soaking in water (or to a certain extent
in clove oil or turpentine).
" One advantage of this process is that it is not necessary
that the objects should be previously hardened with so much
care as is necessary for paraffin-imbedding."   Selenka found
the process applicable for siliceous and calcareous sponges, thy
for worms, and for embryos of the fowl.                          For
250. Albumen imbedding methods have the following              pt
common points. They are methods of imbedding by the wet
way; that is, they do not require that the objects should be
dehydrated and treated with chloroform, essential oils, ether,
or other solvents of paraffin or collodion. The tissues retain
their natural fatty and aqueous constituents, and this, from
a histological point of view, is in many cases an important
advantage. Another good quality of these methods is that
they allow of the cutting of very thin and perfect sections
from the most friable objects. Their great defect is that no
means have been found for removing the mass from the sec-
tions when cut. Its presence in the tissues renders subse-
quent staining extremely difficult or impossible (as the albu-
men takes on a stain with most of the usual colouring agents,
and does not always yield it up on washing out), and, which
is worse, produces confusion in the microscopic images and        i
introduces causes of error into the interpretation of them.

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