HANGING-DROP PREPARATIONS 129 



their growth and development, is by the use of hanging 

 drop preparations. To prepare a hanging drop, a ring of 

 vaseline is painted round the margin of the hollow of a 

 hollow-ground slide (or other cell, see below). A cover- 

 glass is sterilised by flaming in the Bunsen, care being 

 taken not to heat sufficiently to melt it. A droplet of 

 some sterile fluid medium water, broth, wort, sugar 

 solution, etc. is then placed in the centre of the cover- 

 glass with a sterile platinum loop. This droplet is then 

 inoculated with the organism which is to be observed, 

 care being taken not to add too many organisms a few 



FIG. 22. Hanging-drop preparation. 



isolated organisms and small groups in each field is what 

 should be aimed at. The vaselined cell is now taken and 

 turned over, so that the ring of vaseline is downwards, 

 and is then applied to the cover-glass, in such a way that 

 the droplet is situated in the middle of the hollow, but not 

 touching the slide at any point. The cover-glass adheres 

 to the slide by means of the vaseline, and on quickly 

 inverting the whole, so that the fluid has no time to run, 

 it will be found that the droplet is hanging from the 

 under surface of the cover- glass in a cell which is hermeti- 

 cally sealed by the vaseline, and evaporation is thus 

 rendered impossible (Fig. 22). Such a preparation, in 

 fact, can be kept for a week or ten days in a warm incubator 

 without drying up. Great care must be exercised in 

 examining a hanging-drop specimen microscopically, 

 especially with the immersion lenses, for the slightest 

 pressure breaks the unsupported cover-glass. It often 

 saves time first to centre the drop with the low power 

 before examining with the immersion lens ; an ink or 

 pencil dot at the margin of the drop aids focussing. The 



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