MODE OF FIXING COVER-GLASS. 61 



paratiori may be made from it by scraping the epi- 

 thelial surface as before and mounting the product 

 in iodized serum. The preparation so obtained is 

 similar to that already seen of the fresh epithelium, 

 but the separated cells are much more numerous, 

 and at the same time more perfect, for, the inter- 

 cellular substance which unites them being softened 

 and dissolved by maceration in the serum, the cells 

 are more readily obtained free. 



The snip which was placed in osmic acid must be 

 left for two days ; it is then placed in water for an 

 hour or two to remove the excess of the acid. To 

 examine the tissue remove a minute portion and 

 break it up as finely as possible with needles in a 

 drop of glycerine on a slide. The cells will be found 

 to exhibit the same general characters as regards 

 form and appearance, but they are stained of a dark 

 gray color, and in consequence appear for the most, 

 part much more distinct. Any fatty particles which 

 they may contain are colored intensely black. This 

 preparation possesses the advantage over the others 

 that it can be preserved. All that is further neces- 

 sary is to apply a little fixing cement of some sort 

 around the edges of the cover-glass. 



The kind of fixing cement used for preparations 

 mounted in glycerine is immaterial; a solution of hard 

 Canada halsam in chloroform, dammar varnish, gold-size, 

 or Brunswick black may be employed with almost equal 

 advantage; but it is important to see more particularly 

 to two points before applying it. The first is that there 

 shall be just enough glycerine in the preparation to fill 

 up the space between the cover-glass and slide, neither 

 too much nor loo little; any excess must be removed 

 with blotting-paper. At the same time one or two small 

 air-bubbles in the glycerine do no harm, unless they are 

 so situated as to obscure the tissue; on the contrary, 

 by yielding and becoming somewhat compressed when 

 the glycerine tends to expand from being exposed (in 

 summer, for instance) to a warmer temperature, they 

 may tend to prevent that fluid from bursting its bonds 

 6 



