Microtome Sectioning of Material in Paraffin 45 



ers and teaching technicians completely independent of seasonal con- 

 ditions. 



HARDNESS OR BRITTLENESS OF THE MATERIAL 



If the above precautions are observed and satisfactory sections 

 and ribbon are not obtainable from dry blocks of tissues, try the 

 warm-water treatment. Plant tissues embedded in wax are not im- 

 pervious to warm water. If the cutting face of a block of embedded 

 tissues is trimmed to expose the tissues and soaked in water, the 

 paraffin becomes translucent, water penetrates the tissues and renders 

 many hard or brittle subjects soft enough to permit the cutting of 

 excellent sections. Moimt a specimen on a metal disk or on a block 

 of plastic, trim as above, put into a beaker of water, and keep in a 

 35 to 40°C. oven for 12 hr. Objects mounted on wood blocks should 

 be inverted in a vial of water, so that the tissues are submerged. The 

 extent of softening shoidd be tested after 12 hr. by cooling the 

 material to proper cutting temperature and making trial sections. If 

 the tissues are not soft enough, return to the oven for another 12-hr. 

 interval, and test again. If a drop of safranin is added t® the water 

 in which the tissues are soaked, the penetration of the dye provides a 

 good index of the depth of water penetration. Some materials, 

 especially improperly infiltrated tissues, crumble and break out of 

 the paraffin with this treatment. After treatment, material cannot be 

 returned to dry storage because the wet tissues become disorganized 

 on drying. If the hot-water method does not yield sections, the mate- 

 rial is probably too hard to cut by the paraffin method. 



The Operation of the Rotary Microtome 



Having studied the foregoing discussion of some general factors 

 that influence paraffin sectioning, we may turn to the specific opera- 

 tions involved. The operation of the microtome can be learned best 

 by observing an experienced worker. Study the diagrams furnished 

 by manufacturers, and examine your particular instrument with a 

 view to understanding the operating principle and interaction of its 

 parts. Some general suggestions are applicable to the operation of 

 most types of instrument. With the tissue carrier at the upper limit 

 of its travel, and the knife removed or at a safe distance from the 

 path of travel of the tissue carrier, clamp the mounting block bearing 

 the tissues into the object clamp. Manipulate the universal joint of 

 the clamp until the forward face of the trimmed paraffin block, or 

 the desired plane of the sections, is parallel to the knife-edge (Fig. 



