INTRODUCTORY. 5 



the clearing medium,, and soaked until thoroughly saturated 

 in the imbedding medium. This is, for small objects, gene- 

 rally paraffin, liquefied by heat, and for large objects generally 

 a solution of collodion or " celloidin." The imbedding medium 

 containing the object is then made to solidify, as described 

 in the chapter on imbedding processes, and sections are made 

 with a microtome through the imbedding mass and the in- 

 cluded objects. The sections are then mounted on a slide by 

 one of the methods described in the chapter on Serial Section 

 Methods, the imbedding material is removed from them (in 

 the case of paraffin), they are stained in situ on the slide, 

 dehydrated with alcohol, cleared and mounted in balsam or 

 damar. Or they may be stained, washed, dehydrated, and 

 cleared in watch-glasses, and afterwards mounted as desired 

 the imbedding medium being first removed if desirable. 



It is not always desirable to remove the imbedding mass ; celloidin sec- 

 tions stain well without being freed from it, and are usually even dehydrated, 

 cleared, and mounted without removal of the mass, which becomes quite 

 transparent in balsam. This plan has the advantage, which is a very im- 

 portant one for large sections, of allowing the sections to remain during the 

 whole of the manipulations protected by a supporting mass that holds all 

 their parts together. 



The plan of staining sections on the slide is of somewhat recent introduc- 

 tion ; before it had been worked out the practice was to stain structures in 

 toto, before cutting sections. And in cases in which structures are suffi- 

 ciently small and permeable to allow of satisfactory staining in this way, 

 and if it be not essential to save time, this plan is sometimes as good as the 

 one described. In this case the object, after having been fixed and washed 

 out, is taken from the water, or while still on its way through the lower 

 alcohols (it should not be allowed to proceed to the higher grades of alcohol 

 before staining, if that can be avoided), and passed through a bath of stain 

 (generally alcoholic borax-carmine or other alcoholic stain) of sufficient 

 duration, then dehydrated with successive alcohols, passed through a clearing 

 medium into paraffin, cut, and treated as above described, the sections in this 

 case being mounted direct from the turpentine, naphtha, or other solvent 

 with which the paraffin is removed. If aqueous staining media be employed 

 (and it is sometimes very desirable for particular purposes to prepare speci- 

 mens with some aqueous stain) the structures should either be stained in toto 

 immediately after fixing and washing out, or sections may be stained on the 

 slide, the objects being passed through successive baths of alcohol of gradu- 

 ally decreasing strength before being put into the aqueous stain (a precaution 

 which will not be necessary for chromic objects (see below, 5) ). 



It was stated in the first edition of this work that " the 

 great majority of preparations are made by fixing either with 



