26 Transactions of the Society. 



the very first : if then we require to isolate one species from the 

 rest, the expenditure of much time is involved. 



For example, to attain this object it was fproposed, in the 

 method of fractional dilution, to add sterilized nutrient fluid until 

 there was an average of less than one germ to each drop of the 

 fluid. If, then, fresh portions of sterilized nutrient fluid be 

 inoculated with a single drop from the diluted mixture, some 

 portions would in all probability receive no microbes, others would 

 receive one or two, and others, again, one or more microbes of the 

 same species. Then the growth of these microbes would give 

 a pure cultivation of a particular species. It is obvious how com- 

 plicated this process is, and how much the result would depend 

 upon chance. 



If, on the other hand, the mixture was left as a mixture, then 

 the door was open to all sorts of conclusions. Some bacteria being 

 unable to develope in the presence of others, or a change of tem- 

 perature, or a change effected by the micro-organisms in the 

 nourishing soil, allowing one form to predominate over another, 

 the idea could arise that the various kinds of bacteria were but 

 developmental forms of one and the same micro-organism. 

 Further, very probably contamination of such cultivations led to 

 the belief in the transformation of a harmless into a pathogenic 

 bacterium. 



In the case of solid cultivating media, on the other hand, the 

 possible contamination of the nourishing ground by the gravita- 

 tion of germs from the air is guarded against, not by elaborate 

 apparatus or ingenious devices, but by the simple fact that test- 

 tubes, flasks and other vessels can be inverted, and are inoculated 

 from below. 



The great secret of success in Koch's methods of cultivation 

 consists in that we are able, from a mixture of micro-organisms, 

 to isolate the individual species and establish a pure cultivation of 

 each distinct form. By the same method, which is remarkable for 

 its simplicity, if by any possibility contamination has occurred, we 

 can separate the adventitious microbe and regain a pure cultivation. 



This is accomplished in the following manner. A test-tube 

 containing sterilized nutrient gelatin is warmed, and the lique- 

 fied jelly is then inoculated with a platinum needle from the 

 mixture of bacteria, in such a way that the individual micro- 

 organisms are distributed throughout the liquid medium. The 

 liquid is then poured out upon 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 develope 

 individuals of their own species. In this way colonies are formed, 

 each possessing its own characteristic biological and morphological 

 appearances ; if an adventitious germ fall upon the cultivation 



