TEXT-BOOK OF BACTERIOLOGY. 235 



two weeks longer in the incubator, the culture is brought to the 

 height of development. It appears then as a thick, crusty skin of 

 grayish- white color, dry and lustreless, extremely brittle, joined 

 together by numerous little scales, flakes, and nodules. A large 

 quantity of condensation-water always gathers at the dependent 

 parts of test-tubes, and the tubercle bacilli cover this water as a 

 film consisting of single lamella? without even projecting into the 

 depth of the fluid and without dimming or altering the latter in 

 any manner. 



It is a matter of course that by careful transmission upon fresh 

 media artificial cultivation may easily be continued and kept up. 

 We now have a flourishing glycerin-agar culture of tubercle bacilli, 

 which has descended in uninterrupted succession as the one hundred 

 and seventh generation from the first Koch's blood-serum prepara- 

 tion. And with this stately number of ancestors they have pre- 

 served almost unchanged all the qualities of their progenitors : they 

 are just as fit for infection as those and assume specific staining 

 exactly in the manner above stated. 



Such a success can, it is true, be obtained only by using special 

 precaution. Attention is, therefore, called to a few little manipula- 

 tions and measures applied with advantage in transmissions. 



The inoculating matter must be firmly rubbed and pressed in. 

 A shank of very strong platinum wire may be used, but should be 

 thoroughly sterilized in the flame every time before using. 



Having left the tube thus prepared to itself in the incubator, it 

 will soon be seen (unless an unusually well- working thermostat is 

 employed) that an evaporation of the condensation water, a dry- 

 ing up of the culture medium, is taking place, causing a stunted de- 

 velopment of the culture. Attempts to prevent this may be made 

 by drawing small and tightly-fitting rubber caps over the cotton 

 plugs; but while doing so we incur another danger. A kind of 

 moist chamber is formed under the rubber cover; the spores of 

 moulds attached to the plug almost always begin to germinate and 

 shoot mycelium threads through the fibres of the cotton; on the 

 surface of the glycerin-agar, where the tubercle bacilli should ap- 

 pear, there will shortly be seen a layer of moulds. 



We must, therefore, free the cotton from these undesirable in- 

 mates ; this is best done by clipping the plug in the test-tube with 

 scissors after inoculation and by burning the surface in the flame 

 until it is carbonized and turns black. Now carefully drop on the 

 top of the plug one or two drops of a 1 : 1,000 sublimate solution, and 

 finally draw over it the rubber cap which had previously been 

 lying in sublimate. 



