./ 



142 MICROSCOPIC TECHNIQUES 



with absolute alcohol, clearing in xylol, and embedding in paraffin 

 in the usual fashion. But Scott was quick to point out that the 

 advantages of freezing-drying the tissue are particularly important 

 in studies of this nature and hence freezing-drying is the procedure 

 of choice. 



Paraffin sections, 3-5 ^ thick, yield the most satisfactory cyto- 

 logical details. The sections are placed directly on ordinary glass 

 slides of good quality and no adhesive is required to make them 

 adhere to the glass. While the presence of water is scrupulously 

 avoided, a drop of absolute alcohol or liquid petrolatum may be 

 employed to flatten the sections if necessary (Policard and Okkels, 

 1930). If alcohol is used, it is allowed to dry; if petrolatum, it is 

 drained from the slide before the sections are incinerated. The 

 greatest care must be exercised at all times to avoid contamination 

 with dust. Absolutely clean paraffin must be used; and the slides 

 should be washed in distilled water repeatedly, rinsed with filtered 

 alcohol, dried with a clean lint-free cloth, and stored in a dustproof 

 container. 



It is good practice to cut serial sections using alternate ones for 

 incineration and the others for controls to be stained and mounted 

 in the usual manner. Scott ( 1937) pointed out that the use of the 

 cold knife for sectioning, as recommended by Schultz-Brauns ( 1931) , 

 is undesirable since condensation of moisture on its surface may 

 result in some wetting of the tissue. 



2. The Incineration Furnace 



The furnace used for the incineration of tissue sections is simply 

 a quartz tube electrically heated by windings of resistance wire. 

 Ordinary laboratory muffie furnaces can be used if their tempera- 

 tures can be properly regulated and if sufficient care is taken to 

 protect the slides from possible contamination inside the furnace. 



Scott (1937) constructed a very convenient furnace capable of uni- 

 form and reproducible performance. It consists of a quartz tube 24 

 in. long that is wound with three separate 600 watt heating units 

 and the whole covered with asbestos insulation. Each heating unit 

 is controlled by a 44 ohm, 3.2 amp. rheostat. The slides are slowly 

 moved through the furnace tube on quartz slabs by means of an 

 electric motor operating through a speed-reducing worm gear 



