22 FIXATION PROCEDURES 



it consists of metal tubing, and, as the water vapor is not be- 

 ing pumped into a trap at the low-temperature stage, it has 

 not been necessary to incorporate a diffusion pump, since a 

 two-stage rotary pump can do all that is required. The speci- 

 men temperature is defined by a thermostatic device, and the 

 temperature may be checked by a thermocouple which is asso- 

 ciated with each thermostat. As the apparatus does not in- 

 volve mechanical refrigeration, it can run indefinitely without 

 overhauling. Since there is no load thrown on the pump, it is 

 possible to have a number of drying tubes separately linked 

 to the same pump. We are at present using a system of four 

 separate drying tubes, each of which contains two or three trays 

 of specimens. Each drying tube can be opened and shut inde- 

 pendently of the other tubes, so that material can be processed 

 in considerable quantity. 



Provided that the specimen is not to go into an aqueous solu- 

 tion or any other highly polar solvent such as glycerol or formic 

 acid, the specimen may be embedded, sectioned, and examined 

 without any further treatment. We customarily allow the 

 specimen to come up to room temperature in the drying chamber, 

 then place it in an infiltration unit containing degassed wax. 

 The apparatus is evacuated and the wax melted and run onto 

 the specimen, which may then be embedded and sectioned by 

 the usual procedure. Provided that the specimen is allowed to 

 come to room temperature, we have not encountered any diffi- 

 culty with condensation of water vapor from the air as the 

 dried specimen is transferred from the drying unit to the infil- 

 tration unit. It may be necessary under conditions of high 

 humidity to warm the specimen to above room temperature be- 

 fore transfer. If so, the specimen can readily be warmed to a 

 sufficient degree by the heating unit of the drying chamber. 

 Alternatively, the drying tray may be replaced by a bath con- 

 taining a little solidified degassed wax. When the drying is 

 completed, the temperature of the wax can then be raised 

 slightly above its melting point by the heating unit of the dry- 

 ing chamber; infiltration of wax then occurs without moving 

 the specimen. 



If the material to be studied must be subjected to aqueous 

 or other highly polar solvents, a fixative procedure must be 

 used after freeze-drying. The reason for this is that freeze-dry- 



