PREPARATION OF NUTRIENT MEDIA. 43 



air-tight india-rubber cap is drawn over those test- 

 tubes which are not required for immediate use. 

 Before this, however, certain precautionary measures 

 must be taken in order to insure the safety of the 

 contents of the tubes. As has before been mentioned, 

 bacteria cannot pass through a stopper of cotton- 

 wool ; this is, however, not the case with the mould 

 fungi. 



It is true that the germs of these fungi, as well as 

 those of the bacteria, are arrested by the cotton- 

 wool, and that under ordinary circumstances they 

 are unable to develop on account of the lack of 

 moisture. Bat if an india-rubber cap is placed over 

 the cotton-wool, in order to retain the vapour of the 

 condensation water, the cotton-wool becomes mois- 

 tened by this vapour, and thus becomes a suitable 

 medium in which the spores of the mould fungi can 

 develop. These fungi then grow, pushing their long 

 thin threads through the cotton-wool, until they reach 

 its free under-surface, when they bud off spores, 

 which fall into the blood serum, and develop in it. 

 This only occasionally occurs with bacteria, as they do 

 not form such firm threads, and so are not so well able 

 to push their way through the resistant cotton-wool. 

 It is true that the plugs of cotton-wool were sterilised 

 in the hot-air steriliser; but whilst the tubes are being 

 filled, germs may fall out of the air upon them, or 

 may be deposited upon them by the fingers of the 



