100 Histology and Embryology . By C. S. Minot. 
Coagulated gum renders the majority of organs of a pleasant con- 
sistency for cutting. 
Instead of gum, paraffin may be made to permeate the tissues, 
in the way already described in detail in the article on the sledge 
microtome, in the April number of the ‘ American Naturalist.’ 
All acids produce in albumen chemical changes, which, without 
withdrawing the water, cause coagulation. There are some which 
are admirably suited for hardening agents. Foremost among these 
is chromic acid, first introduced by Hannover, in 1841, from 
motives of economy. It is employed in solutions of two-fifth 
parts for one thousand parts water. Very large quantities must 
be used — weak solutions at first, to be gradually replaced by 
stronger and stronger ones. If its action is kept up too long the 
objects become brittle and are then worthless, for every section 
crumbles to pieces as soon as made. Chromic acid is particularly 
useful in studying nervous tissues, organs of sense, and other 
unusually delicate tissues. Its action is very slow : thus the spinal 
cord of a large dog or a man requires at least six weeks or two 
months. Chromic acid is also admirable for preparing very young 
and frail embryos or eggs. There are many other agents which 
are sometimes used for hardening, but it is not deemed appropriate 
to enumerate here any but the two principal and most useful ones, 
alcohol and chromic acid. 
After the proper degree of hardness has been produced, if the 
piece to he cut is large enough, it may be held in one hand and cut 
with the other without more ado. When, however, we have to 
deal with something too small and delicate to be held in the hand, 
it is necessary to have recourse to some method of imbedding. 
Paraffin will usually be found the most convenient substance for 
this purpose, especially when mixed with one-tenth of its weight of 
the best hogs’ lard. The most satisfactory process of imbedding in 
paraffin we have elsewhere described.* 
On some accounts transparent soap is to be highly recom- 
mended. The best quality, containing no glycerine, must be 
chosen, then shaved into small bits, and warmed with half its 
volume of alcohol (as compared with it before it was cut up) until 
it is entirely dissolved ; the specimen to be imbedded is then sus- 
pended in the warm mass by a fine thread and left for twenty-four 
hours. The soap does not become hard until the alcohol evaporates 
from it ; the less alcbhol, therefore, put in originally, the better. 
The soap ought to remain perfectly clear, enabling one to see the 
imbedded specimen within, so that it can easily be observed exactly 
in what plane every section is made, which is not possible when 
paraffin or wax is used. The sections, when made, if cut in soap, 
must be put in alcohol, if from paraffin, in spirits of turpentine, to 
dissolve out the remains of the imbedding mass. 
* ‘American Naturalist, ’ Apiil 1877, p. 208. 
