i6 



proof of isolation is not convincing. In artificial culti- 

 vation advantage is taken of the fact that the bacteria re- 

 produce indefinitely, while the animal tissues of course do 

 not. A drop of blood or pus, or a particle of tissue con- 

 taining the bacteria, is placed in an appropriate nutrient 

 medium in a flask, tube, or other receptacle. In a few 

 hours or days the organisms have become diffused, by 

 virtue of their rapid multiplication, throughout the entire 

 liquid. A drop, or fraction of a drop, of this fluid con- 

 taining bacteria, is then transferred to a second vessel 

 similarly prepared ; after the growth of the organisms in 

 this, a minute quantity is transferred to a third flask, and 

 so on indefinitely at the will of the operator. In this 

 way the bacteria can be practically isolated from the ani- 

 mal tissues introduced with them into the first culture 

 vessel ; and the effect of inoculation from the tenth, 

 twentieth, or thirtieth successive culture cannot be reason- 

 ably ascribed to the unorganized constituents therein 

 contained. But it is evident that in order to attribute 

 the effect of such inoculation to that particular bacterial 

 species contained in the diseased animal, one must be 

 absolutely certain that no other variety has obtained a 

 foothold in the cultures that the original bacteria are 

 isolated not only from the animal tissues a compara- 

 tively simple matter but also from all the other varie- 

 ties of bacteria which seem omnipresent. And just 

 here is the difficulty which has been until recently almost 

 insuperable ; here is the possible source of error which 

 weakens materially some brilliant deductions from ex- 

 perimental work; and this possibility of error is the basis 

 of the general criticism which Koch urges against Pas- 

 teur's work a criticism which, as is evident from a com- 

 parison of methods, is not without foundation. Although 

 numerous modifications of culture methods have been 

 employed, all may be grouped in three general classes. 

 The first, the earliest, and by far the worst, is cultiva- 

 tion in flasks, tubes, or other vessels containing the 

 nutrient liquid, usually in large quantities one or more 

 ounces. This method, thanks to its adoption and re- 



