xxxii INTRODUCTION 



the number of sections required, as well as the purpose for 

 which they are intended. The olyect has in any case to be 

 supported by impregnating it with some homogeneous sub- 

 stance which is firm enough without being brittle or too 

 hard. The freezing method is the simplest of all, but it is 

 not easily applicable when a large number of sections are 

 required mounted in consecutive series, nor when very thin 

 sections are required. The tissue to be cut is first soaked in 

 gum-water, then placed on the freezing-plate of the micro- 

 tome and cooled by an ether spray or a freezing mixture, and 

 cut when just thawing. 



The simplest and quickest imbedding method consists in 

 soaking fresh tissue in acetone, whence it may be transferred 

 direct to paraffin. More usually the specimen, which may 

 be stained previously, is dehydrated by soaking in absolute 

 alcohol ; the alcohol is then removed by soaking in turpentine, 

 xylol, or, better, oil of cedar wood, and the object is then 

 transferred to paraffin kept just melted in a water-oven, the 

 temperature not being allowed to rise above 60 C. Paraffin 

 melting at 55 or 56 C. is the best, though a softer paraffin 

 may be used in a cold room, if it be found that the hard 

 paraffin crumbles before the razor. When the object is 

 thoroughly permeated, it is transferred to a small trough of 

 paper with just enough paraffin to form on cooling a block 

 completely enclosing it. It must be cooled as quickly as 

 possible by placing the trough in a dish containing cold 

 water, as the paraffin is liable to crystallise if allowed to cool 

 slowly. A couple of L -shaped blocks of metal, resting on a 

 plate of glass or metal, so as to form a box of adjustable size, 

 may be used in the place of the paper trough. The block 

 containing the object is fixed in the microtome ; the most 

 efficient of all contrivances for this purpose is, perhaps, a 

 grooved metal plate which is simply warmed slightly and 

 pressed against the end of the block, and then cooled quickly 

 as soon as the block has begun to melt. The further pro- 

 cedure depends entirely upon the particular microtome used. 

 The sections are to be mounted as described in Section XI. 



