16 Microscopic Histochemistry 



important. It is always advisable to avoid excessive heat; the 

 temperature of the paraffin oven should not exceed 56°- 

 58 °C. Even at this temperature tissues should not be ex- 

 posed to heat longer than absolutely necessary. The vacuum 

 technique, to be described in the section on enzymes, will 

 cut down considerably the time required for embedding. 



Paraffin sections should be floated on lukewarm water and 

 attached to the slide either without any adhesive or with P. 

 Mayer's egg-white-glycerol mixture. After complete drying, 

 it is advisable to place the slides in the paraffin oven for a 

 few minutes until they melt. The melted paraffin forms an 

 excellent coating on the surface and protects the tissue from 

 the injurious effect of atmospheric oxygen and moisture. 



The sections are dewaxed in xylene and carried through 

 alcohols ( absolute and 95 per cent ) to water as usual. Often 

 it is advisable to protect the tissue with a thin layer of col- 

 lodion. This is done by flooding the slide after the last alcohol 

 with a dilute (about /2 per cent) solution of celloidin in 

 alcohol-ether, shaking off the excess and hardening the mem- 

 brane in 80-95 per cent alcohol. The collodion membrane 

 serves two purposes : ( 1 ) it prevents diffusion of large mole- 

 cules not made insoluble by fixation, and (2) it facilitates 

 the removal of nonspecffic surface precipitates, a by-prod- 

 uct of some histochemical reactions. These precipitates will 

 settle on the surface and can be washed off by dissolving 

 the membrane in alcohol-ether or acetone. 



