PREPARATION OF NUTRIENT MEDIA. 63 



one form to predominate over another, then one 

 might be led to the conclusion that many bacteria 

 were but developmental forms of one and the same 

 micro-organisrn ; while possibly the contamination 

 of such cultures might lead to the belief in the trans- 

 formation of a harmless into a pathogenic bacterium. 

 In the case of solid cultivating media, on the other 

 hand, the chance of contamination by gravitation of 

 germs from the air is avoided by the fact that test- 

 tubes, flasks, etc., can be inverted and inoculated from 

 below. The secret of the success of Koch's method, 

 however, depends upon the possibility, in the case 

 of starting with a mixture of micro-organisms, of 

 being able to isolate them completely one from 

 another, and to obtain an absolutely pure growth 

 of each cultivable species. When sterile nutrient 

 gelatine has been liquefied in a tube and inoculated 

 with a mixture of bacteria in such a way that the in- 

 dividual micro-organisms are distributed throughout 

 it, and the liquid is poured out on a plate of glass and 

 allowed to solidify, the individual bacteria, instead 

 of moving about freely as in a liquid medium, are 

 fixed in one spot, where they develop individuals of 

 their own species. In this way colonies are formed 

 each possessing its own characteristic biological 

 and morphological appearances. If an adventi- 

 tious germ from the air falls upon the culture, it 

 also grows exactly upon the spot upon which it fell, 

 and can be easily recognised as a stranger. To 

 maintain the individuals isolated from one another 



