INTRODUCTORY. 7 



decomposition of organic matter, and its complete removal is 

 the chief condition of permanent preservation. 



It is of course not intended here to suggest that wet methods of prepara- 

 tion should be altogether discarded. They have great value, they are even 

 indispensable, for special ends ; and all that is intended to be suggested is, 

 that they should be regarded not as general but as Special methods. 



4. In the preparation of entire objects or structures that are 

 intact and covered by an integument 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 diffusibility should be em- 

 ployed as far as possible in all the processes. Fixing agents 

 of great penetrating power (such as picrosulphuric 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 carminic acid or haematein 

 stains (for all of which see STAINING AGENTS). Anilin 

 stains are rarely applicable to this class of preparations. 

 Aqueous stains are more seldom used, though there are 

 many cases in which they are admissible, and some in which 

 they are preferable. 



Minute dissections are best done, if necessary, in a drop of 

 clearing agent. I recommend cedar-wood oil for this pur- 

 pose, as it gives to the tissues a consistency very favorable 

 for dissection, whilst its viscosity serves to lend support to 

 delicate structures. Clove oil has a tendency to make tissues 

 that have lain in it for some time very brittle. The brittle- 

 ness is, however, sometimes very helpful in minute dissections. 

 Another property of clove oil is that it does not easily spread 



