HANGING-DROP PREPARATION 145 



any length of time, a ring of oil or vaseline may be painted 

 round the margin of the cover-glass so as to seal it to the 

 slide. 



A simple method for keeping organisms under examina- 

 tion for a lengthened period of time, and of watching 

 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 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 



FIG. 24. Hanging-drop preparation. 



organism which is to be observed, care being taken not 

 to add too many organisms a few isolated organisms 

 and small groups in each field is what is required. The 

 prepared cell is now taken and turned over, vaselined 

 surface 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 

 hermetically sealed by the vaseline, and evaporation is 

 thus rendered impossible (Fig. 24). 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 exer- 

 cised in examining a hanging-drop specimen microscopi- 

 cally, especially with the immersion lenses, for the slightest 



M.B. 10 



