PREPARATION OF SPECIMENS 5 



in the clearing fluid. If the specimens become cloudy 

 or white upon transference to the clearing agent, it is 

 an indication that dehydration is incomplete. In 

 this case the material must be returned to the stronger 

 alcohol mixtures and a longer time allowed for dehy- 

 dration. 



Infiltration. — When the water within and around 

 the cells has been replaced by chloroform, and the 

 cell walls rendered transparent by means of a clear- 

 ing medium, the next step is to introduce melted 

 paraffin into the cavities of the cells and into the inter- 

 cellular spaces. As previously stated, the paraffin 

 serves as a supporting medium and prevents collapse 

 of the specimen during section cutting. Melted 

 paraffin will dialyse through cell walls, but the rate 

 of dialysis can be materially hastened by immersing 

 the specimen in a saturated solution of paraffin in 

 chloroform or xylol. After this treatment the speci- 

 men may be transferred to melted paraffin kept at 

 a temperature just above melting point. The paraffin 

 for this purpose is especially prepared and the several 

 grades obtainable range in- melting point from 45° C. 

 to 55° C. The grade to be employed depends upon 

 the season and the texture of the specimen. In hot 

 weather it is more satisfactory to use paraffin of higher 

 melting point than that used in cold weather. The 

 temperature of the paraffin bath must be kept fairly 

 constant and slightly above the melting point. Infil- 

 tration will be slow or incomplete if the paraffin is 

 too cold. If the temperature of the paraffin bath 

 rises much above the melting point there is great 

 danger of rupturing cell walls. The steps in the 

 infiltration process are as follows: 



