INTRODUCTORY. 9 



as fixation, washing out, staining 1 , treatment with successive 

 alcohols of gradually increasing strength, final dehydration 

 with absolute alcohol, clearing, and mounting in balsam. 

 This method is usually preferred, as a general method, to the 

 wet methods, for the reasons that have been given above 

 ( 2), and for some others, amongst which may be noted the 

 greater transparency given to tissues by mounting them in 

 media of high refractive index, such as balsam. 



In the preparation of entire object* or structures that are 

 intact and covered by an in tegument not easily permeable by 

 liquids, special care must be taken to avoid swelling from 

 endosmosis on the passage of the objects from any of the 

 liquids employed to a liquid of less density, or shrinkage from 

 exosmosis on the passage to a liquid of greater density. This 

 applies most specially to the passage from the last alcohol 

 into the clearing medium. A slit should be made in the 

 integument, if possible, so that the two fluids may mingle 

 without hindrance. And in all cases the passage is made 

 gradual by placing the clearing medium under the alcohol, 

 as above described. Fluids of high di (fusibility should be 

 employed as far as possible in all the processes. Fixing 

 agents of great penetrating power (such as picro-sulphuric 

 acid or alcoholic sublimate solution) should be employed 

 where the objects present a not easily permeable integument. 

 Washing out is done with successive alcohols, water being 

 used only in the case of fixation by osmic acid, or the chromic 

 mixtures or other fixing solutions that render washing by 

 water imperative. Staining is done by preference with 

 alcoholic staining media. The stains most to be recommended 

 are Grenadier's borax-carmine, or one of Mayer's new 

 carniinic acid or haematein stains (for all of which see 

 STAINING AGKNTS). Tar-colour stains are rarely applicable to 

 this class of preparations. Aqueous stains are more rarely 

 indicated, though there are many cases in which they are 

 admissible, and some in which they are preferable. 



8. Minute Dissections. These are best done, if necessary, 

 in a drop of clearing agent. I recommend cedar- wood oil 

 for this purpose, as it gives to the tissues a consistency very 

 favourable for dissection, whilst its viscosity serves to lend 

 support to delicate structures. Clove oil lias a tendency to 



