Wholemoiints in Gum Media 



General Principles 



Nature of the Process 



The preparations which have been de- 

 scribed in the last two chapters are those 

 in which the specimen is sealed in a pre- 

 servative fluid. These mounts, as will be 

 readily understood by anybody who has 

 read the chapters, are difficult and labori- 

 ous to prepare, so that most slides are 

 made in a mounting as distinguished from 

 a 'preservative medium. A mounting me- 

 dium,, used in this sense, is one which itself 

 hardens and holds the coverslip in place 

 while at the same -time preserving the 

 object contained in it. Mounting media 

 may be divided into two large groups: 

 first, those which are miscible with water; 

 second, those which are not miscible with 

 water, so that some initial treatment must 

 be given to most objects before they are 

 mounted. The media miscible with water 

 are in themselves divisible into two types: 

 first, those which are liquid at room tem- 

 perature (dealt with in this chapter), 

 second, those which are solid at room tem- 

 perature and must be melted before they 

 can be used. Water-soluble media are all 

 colloidal dispersions of various materials, 

 the colloids being in the sol phase for the 

 media described in this chapter, and in the 

 gel phase for the media described in the 

 next. 



Types of Gum Media Employed 



Many formulas for mounting media of 

 the sol-colloidal type are given in Chapter 

 27 under the heading M 11.1. They are 

 dispersions of either natural or synthetic 

 gums in water and must, therefore, depend 

 for their hardening upon the evaporation 



of moisture from the edge of the coverslip. 

 Were this to continue for an indefinite 

 period, the media would naturally harden 

 and crack; hence, most contain either 

 glj^cerol or sorbitol to impart hygroscopic 

 qualities. The prototype of all these media 

 is Farrants', which is a simple dispersion 

 of gum arable in water to which has been 

 added a small quantity of glycerol to- 

 gether with a preservative. All media de- 

 rived from this follow the same pattern, 

 differing mostly in the quantity of glycerol 

 included and in the nature of the preserva- 

 tive selected. The fundamental objection 

 to gum-arabic media is that it is difficult 

 to obtain a pure sample of the gum, and 

 one has to go through a wearisome process 

 of filtration to avoid having the mount 

 filled with sand grains and pieces of stick. 

 Another objection to this type of medium 

 is the low index of refraction, which 

 leaves objects mounted in it relatively 

 opaque when they are examined by trans- 

 mitted light. This difficulty is overcome in 

 Berlese's medium, to which chloral hy- 

 drate is added in considerable quantities 

 with a view to increasing the index of re- 

 fraction. The ordinary media of the 

 Farrants' type have an index of refraction 

 just over 1.3, while Berlese's medium and 

 its modifications have indices of refraction 

 as high as 1.47. Very few synthetic sub- 

 stitutes for water-soluble gums are avail- 

 able, the most promising at the present 

 time being polyvinyl alcohol, which is 

 used in the media of Downs, and of Gray 

 and Wess. It is probable that the recent 

 appearance on the market of water-soluble 

 cellulose derivatives (for example, car- 

 boxymethyl cellulose) may lead to the 



42 



