BROWN: Woops FOR MICROSCOPIC STUDY 135 
The presence of such a film was indicated, though not certainly 
proven, by refraction tests. Another possible agency of protection 
may consist in a thin and presumably impermeable organic mem- 
brane closely applied to the surface of the crystal. Such a mem- 
brane may be detected by zinc chlor-iodide or by the haematoxylin 
stain. Also, a thicker mucilaginous membrane is usually visible 
outside the inner thin membrane. The resistant character of 
these organic coverings is indicated from the fact they may be 
little if at all acted upon by Schultze’s solution. 
As soon as the tissues have been sufficiently softened to cut. 
well, they should be washed in running water for about four days 
to remove all acid. An arrangement like that shown in Fic. 3 
is recommended both for this purpose and for washing the sections 
at a later stage. The blocks are placed in a short jar the top of 
which is covered with cheese-cloth through which the pointed end 
of a glass siphon tube is pushed. The long arm of the siphon 
should extend to the bottom of the jar. The short arm takes its 
water from a somewhat taller jar placed beneath a running tap. 
After washing, the blocks should be covered with glycerine where 
they may remain until needed for sectioning. The effect of the 
glycerine is such that any tissues which have become too brittle, 
in a few hours, become sufficiently flexible to cut well with the 
microtome. 
D. IMBEDDING 
Celloidin method.—Certain woods of peculiar structure, like 
Pisonia, in which portions are composed of very soft unlignified 
tissues, and other woods, in case it is desired to cut extremely thin 
sections, must be imbedded in celloidin before cutting. In 
imbedding, the general procedure followed was that described by 
Plowman (’04). Solutions of Schering’s celloidin in a mixture 
composed of equal volumes of ether and absolute alcohol may be 
prepared in 2 x 8 cm. shell vials in concentrations of 2, 4, 6, 8, 10, 
12, 14, 16, 18, and 20 per cent. The blocks to be imbedded are 
first treated in hydrofluoric acid and washed as above described. 
Next, in the usual manner, they are gradually transferred to 100 
per cent alcohol. During the latter stages of this dehydration 
process, they are left two days in each alcohol, and the absolute 
alcohol is changed at least once. They are now transferred to a 
