THE PARAFFIN METHOD 
109 
Spirogyra, however, may be wrapped in several thicknesses of news¬ 
paper, placed in a botany can and brought into the laboratory. 
Before fixing, it should be placed in water for half an hour. Such 
forms suffer more from lack of air when placed in a bottle or a can 
than from lack of water when wrapped in wet newspaper. Branches 
with developing buds may be brought in and kept in water. Cones of 
the cycad, Ceratozamia , sent from Jalapa, Mexico, have arrived in 
Chicago with cell division still going on at a rapid rate. But such 
cases are extremes; as a rule, take the killing and fixing fluids into 
the field. 
Always have the material in very small pieces, in order that 
the reagents may act quickly on all parts of the specimens. Pieces 
larger than cubes of 1 cm. should be avoided whenever possible. 
While one sometimes needs sections 2 or even 3 cm. long, it is not 
likely to be necessary to fix pieces more than 4 or 5 mm. in thickness. 
For Very fine work no part of the specimens should require the 
reagent to penetrate more than 1 or 2 mm. 
For fixing agents of the chromic-acid series, the volume of the 
reagent should be about 50 times that of the material. 
Fixing agents with alcohol as an ingredient will fix a larger pro¬ 
portion of material. It must be remembered that the water, which 
is always present in living tissues, weakens the fixing agent. 
The time required for fixing varies with the reagent, the character 
of the tissue, and the size of the piece. About 24 hours is a com¬ 
monly recommended period for chromic-acid solutions, but 2 or even 
3 days will do no harm. 
Directions for making and using the various fixing agents are 
given in the chapters on “ Reagents” (chaps, ii, xxix). 
WASHING 
Nearly all fixing agents, except the alcohols, must be washed out 
from the material as completely as possible before any further steps 
are taken, because some reagents leave annoying precipitates which 
must be removed, and others interfere with subsequent processes. 
Aqueous fixing agents with chromic acid as their principal ingredient 
are washed out with water; aqueous solutions of corrosive sublimate 
are also washed out with water; but alcoholic solutions should be 
washed out with alcohol of about the same strength as the fixing 
agent; picric acid, or fixing agents with picric acid as an ingredient, 
