394 The Study of Zoölogy in Germany. [July, 
of the best quality, and when used always drawn towards the 
body, while the surface, which looks downward in cutting, must 
be flat. The edge must be perfect, the slightest notch being suf- 
ficient to tear a section to pieces, and so sharp that a human hair 
can be split with it. The sections themselves must be as thin as 
possible. 
Since all parts of the body, with few exceptions, such as the 
skeleton, etc., are soft and permeated by water, besides possess- 
ing great elasticity, they cannot be cut in their natural condition ; 
it becomes necessary, therefore, to harden the organs. Now pro- 
toplasm is the main constituent of cells, and itself consists chiefly 
of albumen. This substance can be coagulated by the action of 
various agents, some of which can be applied to the tissues 
without injuring them, to produce a coagulation of the albumen _ 
in its natural form within the cells. 
Alcohol is one of the most valuable agents for this use. It 
produces its effect by its strong affinity for water, which it can 
withdraw from the tissue, thus causing the albumen, which re- 
‘quires an abundance of water to maintain its semi-fluid state, to 
solidify. It may be employed for the majority of tissues with 
perfect success. The volume of alcohol should be from twenty 
to thirty times that of the object to be hardened; weaker alco- 
hol, say of eighty per cent., should be used first ; after a sojourn 
of an hour or two, or even longer, if large, the object may be 
transferred to stronger (ninety-six per cent.) spirit and there left 
for twenty-four hours, more or less, according to the size of the 
piece. The great difficulty in the use of alcohol is to prevent 
the shrinkage which naturally follows upon the abstraction of 
the water from the tissues. This may be avoided by using first 
weak, and then strong, and finally very strong spirit. In some 
cases the action is not even then sufficient, and recourse must be 
had to absolute alcohol, which generally produces the desired 
result. shies 
When even that does not succeed the specimens may be put m 
picric acid (concentrated cold aqueous solution) for twenty-four 
hours, then in a syrupy solution of gum arabic for twenty-four 
hours, and finally in strong alcohol again for the same length 
of time. The picric acid removes the alcohol, and allows 
gum to penetrate the object, within which it is finally coagulated 
by the last dose of spirit. The sections when made must be 
left in water for a day, to dissolve out the gum which they still 
contain, and which renders them quite opaque. A very few 
