THE USE OF CULTURE MEDIA. 53 



Cotton-wool plugs are universally used for protecting the 

 sterile contents of flasks and tubes from contamination 

 with the bacteria of the air. The contained air passes 

 through the plug during sterilisation ; what passes back on 

 cooling is filtered free of germs by the wool. A medium 



FIG. 12. Apparatus which FlG. 13. Tubes of media, 



may be used for filling tubes. a Ordinary upright tube. b. Sloped tube. 



The apparatus explains itself. c . "Deep" tube for cultures of 



The india - rubber stopper anaerobes, 

 with its tubes ought to be 

 sterilised before use. 



thus protected will remain sterile for years. Whenever a 

 protecting plug is removed for even a short time, the 

 sterility of the contents is endangered. It is well to place 

 the bouillon, gelatine, and agar media in the test-tubes 

 directly after filtration. The media can then be sterilised 

 in the test-tubes. 



