428 HISTOLOGICAL TECHNIQUE. 



Methods of Dehydration. 



The final manipulation in nearly all the methods for staining 

 described above is a washing of the sections in water. This water 

 must be removed before permanent mounts can be made. Dehy- 

 dration is accomplished by treating the sections with alcohol. If 

 they are impregnated, or have been embedded in collodion or eel- 

 loidin, they must not be dehydrated in absolute alcohol, as that dis- 

 solves the collodion. In such cases 95 per cent, alcohol is employed, 

 the sections being treated with two baths of alcohol. When sections 

 have been stained with carmine a contrast-stain may be obtained by 

 adding a few small crystals of picric acid to the first dish of dehy- 

 drating alcohol. The excess of picric acid is then removed by the 

 alcohol in the second dish. Absolute alcohol may be used for dehy- 

 dration when the sections have not been embedded in collodion or 

 celloidin. 



When anilin-dyes have been used to stain sections it must be 

 borne in mind that alcohol not merely dehydrates, but also differ- 

 entiates the stain. If the sections are left too long in the alcohol, 

 they may lose more color than is desired. 



Sections that are to be mounted in glycerin or glycerin-jelly 

 require no dehydration, but can be mounted directly from water. 



Methods of Clearing. 



Clearing is necessary when specimens are to be permanently 

 mounted in Canada balsam or dammar. Its object is to impreg- 

 nate the section with some liquid that will drive out alcohol and 

 also be miscible with the resin used for mounting. Of these clear- 

 ing-agents there is a large number, from which a choice must be 

 made according to the method of embedding that has been employed 

 and the nature of the dye with which the tissues have been stained. 

 Clearing-agents also differ in their miscibility with water, some 

 requiring dehydration with absolute alcohol, others clearing well 

 when 95 per cent, alcohol has been used for dehydration. 



1. Xylol. This is an excellent clearing-agent when the sections 

 have been well dehydrated with absolute alcohol. It does not 

 injure anilin-dyes. It is, perhaps, the best clearing-agent for 

 sections of tissue stained with borax-carmine before cutting, when 

 no counter-stain is employed. Xylol then both removes the paraffin 

 in the section and clears it. 



