CULTIVATION ON PLATES AND IN TUBES. 55 



solidification has begun to take place an even distribu- 

 tion of the germs in the nutrient medium is no longer 

 possible, neither does the agar-agar become evenly 

 spread over the bottom of the dish. 



The different bacteria which develop in this medium 

 at the ordinary room temperatures grow at very dif- 

 ferent rates. It is possible that on the following day 

 colonies of bacteria may. be apparent to the naked eye; 

 but often one has to wait two or three days,- or even 

 longer, according to the special kinds which happen 

 to be present and to the temperature of the room. In 

 order to convince oneself that the latter plays a most 

 important part, it is only necessary to compare the 

 results obtained during the hot summer weather with 

 those obtained in spring and autumn. 



With the naked eye it is difficult to distinguish the 

 bacteria colonies from small air-bubbles ; they occur 

 as a rule in isolated positions, as the solidification of 

 the gelatine happened to fix the germs. As they 

 grow the different kinds may be distinguished from 

 one another by a great number of delicate characteris- 

 tics. 



In gelatine cultures it is first to be noticed that 

 whilst some sink down somewhat in the gelatine, 

 causing it to liquefy in more or less wide circles around 

 them, with others the liquefaction is a much slower 

 process, and is only to be perceived after some time, 

 and yet others do not liquefy the gelatine at all. A 



