INTRODUCTORY. 7 



a drop of clearing agent being a most helpful medium for 

 carrying out such dissections (see 8). 



At this point the course of treatment follows one of two 

 different roads, according as the object is to be mounted 

 direct in balsam ( 7), or is first to be sectioned (5). 



5. Imbedding, and Treatment of Sections. The objects are 

 now imbedded. They are removed from the clearing medium, 

 and soaked until thoroughly saturated in the imbedding 

 medium. This is, for small objects, generally paraffin, 

 liquefied by heat, and for large objects generally a solution 

 of collodion or " celloidin " (in this latter case the clearing 

 may be omitted and the tissues be imbedded direct from the 

 alcohol). The imbedding medium containing the object is 

 then made to solidify, as described in the chapter on Im- 

 bedding Processes, and sections are made with a microtome 

 through the imbedding mass and the included 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 im- 

 bedding medium being first removed if desirable. 



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. 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 chloroform, xylol, 

 or other solvent with which the paraffin is removed. If aqueous staining 

 media be applied (and this is sometimes very desirable for particular pur- 

 poses) the structures should either be stained in toto immediately after 

 fixing and washing out, or sections may be stained on the slide, the objects, 

 if delicate, being passed through successive baths of alcohol of gradually 

 decreasing strength before being put into the aqueous stain. 



In my opinion it is generally advisable not to stain in bulk material that 

 is intended to be sectioned; by staining it as sections the staining can be 



