METHODS OF CULTIVATION. 157 



ized fluid until there shall be an average of less 

 than one living germ to each drop of fluid. If 

 now we inoculate numerous separate portions of a 

 sterilized culture-medium with a single drop, each, 

 of this diluted stock, it is evident that some por- 

 tions may receive no living seed, others may have 

 germs of two or more species, and others may 

 chance to have one or more germs of a single 

 species. In the latter case, the multiplication of 

 these germs under conditions which excluded the 

 possibility of contamination from without would 

 give us a pure culture of this particular species. 

 So far as the writer is aware this method has not 

 been employed, except in a limited number of 

 experiments made by Lister himself in order to 

 demonstrate its feasibility. No doubt it may be 

 successfully employed, but it would involve a 

 great expenditure of time, and success would 

 probably be the exception and failure the rule, 

 owing to the difficulty of estimating the exact 

 amount of dilution required in the first instance, 

 and because of the element of chance, which is an 

 essential feature of the method. 



The same result is accomplished more expedi- 

 tiously by the method of Koch, the essential fea- 

 ture of which consists in using a solid sub-stratum 

 as the culture-medium, upon which the mixed 

 micro-organisms are distributed. A sufficient quan- 

 tity of gelatine (3 to 5 per cent.) is added ta a 

 suitable culture-fluid to cause the mixture to jellify 

 when cooled. While still warm, this gelatine cul- 



