The Celloidin Method 85 



and must be biiili to specifications. A satisfactory apparatus, described 

 by Chowdhiny (1934), consists ot a section ol iron })ij)C with a 

 threaded fhtnge at each end. Phites are bolted to the flanges, and the 

 upper plate is removable for introducing the specimens. The com- 

 pression chamber is lead lined and is provided with a pressure gauge 

 and a valve to which the pumj) is attached. Chowdhury recommends 

 40% HF and a pressure of 80 lb. He found tliat 1-in. cube blocks 

 of Jiiglaiis regia were adequately softened in 3 days; blocks of 

 Diospyros melanoxylon, an extremely hard wood related to our per- 

 sinmion, required 7 days. The ecjuipment necessary for this method is 

 amply justified if a considerable amount of diagnostic work on 

 timber woods is being carried on. 



Material embedded in celloidin can be cut as soon as the cel- 

 loidin has been hardened in chloroform and the volatile chloroform 

 replaced with the glycerin-alcohol storage fluid. The cutting proper- 

 ties are improved by prolonged storage in glycerin-alcohol. If mate- 

 rials having dark-colored bark and light-colored wood are stored 

 for several years, the storage fluid dissolves coloring matter from the 

 bark and imparts a dark color to the wood. Stained sections from 

 such wood do not have bright, clear colors. The stock of embedded 

 twigs of basswood, for example, shotdd be replaced every 3 to 5 

 years. Incomplete removal of killing fluids or of hydrofluoric acid 

 results in gradual disintegration of stored material. 



Sectioning 



Celloidin sections are usually cut with a sliding microtome. In 

 this type of instrument the material is stationary during the cutting 

 stroke, while the knife carriage slides on an accurate track. An auto- 

 matic or hand-operated feed mechanism moves the tissues upward 

 between cutting strokes. The catalogues of the leading manufacturers 

 contain instructive illustratirjns and descriptions of several types and 

 price classes of sliding microtomes. Small pieces of moderately soft 

 tissues can be cut with a razor blade in a special holder designed for 

 the sliding microtome. The limitations of the razor blade must be 

 determined by trial. Hard materials and large sections must be cut 

 with a microtome knife. Various lengths and weights of knives are 

 available. The method of sharpening a microtome knife is described 

 in Chap. 6. 



Before using the sliding microtome, wipe the track of the knife 

 carriage with an oiled cloth and test the feed mechanism. Clamp the 

 knife firmly into the sliding carriage. Remove a piece of blocked tissue 



