6 INTRODUCTORY. 



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. 



Or. the material may be stained in bulk, before cutting the 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, 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 desir- 

 able), 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 much better controlled, and many excellent stains 

 can in this way be employed that are not available for staining in 

 bulk ; and of course sections can be stained much more rapidly than 

 material in bulk. 



Balsam mounts of which the stain has faded, or which it may be 



/ 



desired to submit to some other staining process, or mount in some 

 other medium, may often with great advantage be re-stained and 

 re-mounted. All that is necessary is to put the slide into a tube of 

 xylol or benzol till the cover falls off (about two days), wash well for 

 some hours in clean xylol, and pass through alcohol into the new 

 stain. Since this was pointed out to me by Dr. Henneguy I have 

 unmounted and re-stained a large number of old preparations, and 

 have succeeded in every case with series of sections mounted on 

 Mayer's albumen, or by the water method. For shellac-mounted 

 series, see E. MEYER, Biol. Centralb., x, 1890, p. 509, or last edition. 



The most convenient vessels, I find, in which to perform the various 

 operations of staining, differentiating, dehydrating, clearing, etc., on the 

 slide, are flat-bottomed corked glass tubes. I have mine made 10 



