52 ESSAYS ON BACTEEIOLOGY. 



agents are different, then, could we but isolate the 

 latter, we might use it at any stage of the disease, and 

 only with benefit. 



Then, too, the question arises whether the agents 

 producing self -limitation and immunity are the same, . 

 and the possibility, if different, of separating and ob- 

 taining them for practical use. We must also ask 

 ourselves whether it is always or usually possible to 

 obtain the desired substances from artificial cultures 

 of the germs. May there not be processes attending 

 the growth of the germs in the living body which are 

 not possible in the artificial experiments? For, as 

 FrsBnkel aptly reminds us, the living tissue is no plate 

 of gelatine and no test-tube. How far this may prove 

 an obstacle to progress is as yet uncertain; but it is 

 certain that it does not necessarily and always forbid 

 success by the artificial method. 



Here we see two schools of experimenters arising. 

 The one excludes the living germ, seeking and using 

 only the sterile germ products. This they do under 

 the belief that, could the problems indicated be solved, 

 and the processes be worked out to exactness by chem- 

 istry, the method would be more accurate and less 

 dangerous. For, thus far, experiments made with 

 even weakened living germs have proven to be accom- 

 panied by more or less risk. 



The other school, acting upon the conviction that 

 artificial culture cannot be relied upon to produce the 

 same results as germ-growth in the living tissues, uses 



