Sectioning Unembedded Tissues 93 



instantly. After 10 niin. in the llitid, rinse the sections in water, and 

 proceed with the staining. 



The preceding methods permit the stndy of living cells, or cells 

 in which protoplasmic details were fixed by reagents. If these methods 

 of sectioning unembedded material fail to give satisfactory results, 

 the materials must be treated by methods which may distort or destroy 

 fine protoplasmic details. Nevertheless, the following methods are 

 useful, within the stated limitations. 



Dry lumber of soft woods such as white pine, basswood, or willow 

 can be sectioned successfully without embeddding. Trim the wood 

 carefully into blocks measuring approximately 1 by 1 by 2 cm., and 

 prepare the blocks by alternately boiling in water and pumping in 

 cold water, until the pieces are thoroughly saturated and sink. Hard 

 woods that cannot be cut after this treatment may be sectioned by 

 one of the following methods. 



The live-steam method of sectioning wood is based on the principle 

 used in the manufacture of veneer. If a jet of superheated steam is 

 directed upon the surface of a block of wood, the surface becomes 

 soft enough to permit the cutting of a thin section. Steam can be 

 generated in a flask, but a safer device is a small steam generator, 

 provided with a pressure gauge, water level gauge, safety valve, and 

 a water inlet. Such generators are obtainable from dealers in chemical 

 apparatus. The steam from the generator passes through a copper 

 tube, in which there is a superheating coil heated by a Bunsen burner, 

 and the superheated steam emerges through the small orifice of a 

 nozzle which can be adjusted over the specimen. Successive sections 

 are cut at experimentally determined intervals of steaming. For 

 additional details and variations of the apparatus refer to Crowell 



(1930) and Davis and Stover (1936). 



- Jeffrey's vulcanization method makes possible the sectioning of 

 extremely hard materials, such as walnut shells. Materials are sealed 

 in a chamber made from a section of pipe, and heated in a dental 

 vulcanizer at approximately 160°C. for 1 to 5 hr. This is followed by 

 treatment in hydrofluoric acid. Materials may be cut unembedded or 

 in celloidin. Because of the special equipment necessary, the 

 procedure is not described here, and interested workers are referred to 

 Jeffrey (1928). 



Sectioning With the Freezing Microtome 



Unembedded tissues that are too soft or fragile to stand up under 

 the impact of the knife can in some cases be frozen and then 



