128 CYTOLOGICAL TECHNIQUE 



Stick the coverslip to the slide by the use of a seal, which prevents 

 movement and also the evaporation of any volatile constituent. 

 Seahng is discussed on p. 133. 



Four mounting media will be described here. They are repre- 

 sentative of the very large number that exist. They have been 

 chosen because they are simple in composition and therefore 

 demonstrate well the principles involved in the process of 

 mounting. The selected examples are these : 



hydrophil 

 non-adhesive (fluid mount) . glycerol (see below) 

 adhesive .... Farrants's medium (p. 129) 



hydrophobe 

 non-adhesive (fluid mount) . methyl salicylate (p. 130) 

 adhesive .... DPX (p. 131) 



Glycerol (glycerine) is a familiar non-adhesive hydrophil 

 medium. It is useful in studies of lipids, which are preserved 

 whether fixed or not. Lysochromes (p. 86) maintain their colour 

 well in this medium, but certain dyes, including mordanted 

 haematein, are not perfectly stable. ^*^ Glycerol is very valuable as 

 a mounting medium for sections of tissues that have been em- 

 bedded in butyl methacrylate and are intended for study, without 

 dyeing, by phase-contrast microscopy. ^^ When preparing tissues 

 for electron-microscopy it is desirable to cut a few sections at 

 about 3 /^ and to examine these by phase-contrast. This shows at 

 once whether the tissue is well enough fixed to make it worth while 

 to cut thin sections and mount them on grids for electron- 

 microscopy. It also helps in the interpretation of electron- 

 micrographs, because the thickness of the section enables one to 

 focus up and down and thus obtain a three-dimensional view. 

 Material that has been fixed in Palade (p. 58), embedded in 

 methacrylate (p. 77), sectioned at about 3 ^, and mounted in 

 diluted glycerol, gives an extraordinarily lifelike appearance 

 under the microscope. A few authors^^^- ^^^ have studied meth- 

 acrylate sections by phase-contrast, but not nearly enough has 

 been done in this promising field. 



It is generally supposed that the fixed proteins of the cell have 



