136 Brown: WooDs FOR MICROSCOPIC STUDY 
mixture of ether and alcohol, in equal volumes, and finally through 
the graded series of celloidin solutions, being left about twenty- 
four hours in each concentration. On reaching the 20 per cent 
solution, a pressure-resisting container should be used. A brass 
case with screw top, such as a microscope objective box, will 
answer the purpose, or the corked imbedding vial may be wrapped 
tightly with strong cord. After adding sufficient solid celloidin 
to thicken the solution as much as possible, it is placed in a paraffin 
oven at 50-60° C. for three days. During this time, the contents 
of the vial are kept under pressure by the confined gases with the 
object of forcing the celloidin more perfectly into the tissues. - 
The imbedded blocks are then removed, hardened in chloroform 
for twelve to twenty-four hours, and placed in a mixture of equal 
parts of 95 per cent alcohol and glycerine until ready for sectioning. 
Paraffin method—Only very soft tissues should be imbedded 
in paraffin. Woody tissues, even after being well softened by 
acid, are likely to become too hard during the process of imbedding. 
in paraffin to cut well with the microtome. 
E. SECTIONING WITH THE MICROTOME 
Unimbedded material.—After softening the tissues as already 
described, the material is ready to section. In cutting sections, 
the blade of the knife should be kept flooded with a 15 per cent 
solution of glycerine in 95 per cent alcohol. Sections are best 
removed from the blade by a fine camel’s hair brush and are 
transferred to water. From blocks prepared as above described, 
cross-sections may be cut as thin as I 0-15 u; radial sections, 8-20 p; 
and tangential sections about 7 4. Where, for special purposes, 
such as the study of the detailed structure of pit membranes, it is 
necessary that the sections be considerably thinner than this, the 
material must be imbedded in celloidin as above described. 
Imbedded material—To section material after imbedding in 
celloidin, the blade may be moistened either with 95 per cent alcohol 
or with the alcohol-glycerine mixture. Sections of woody tissues 
may then be cut to less than 3 in thickness. To obtain very 
thin sections, it is essential not only that the knife be sharp, but 
that it be ground to an alignment sufficiently true to admit of the 
use of the blade at very oblique horizontal inclinations with refer- 
