510 Chapter XVI 



[533] received enormous quantities of the bacillus may die from ailthrax in 

 spite of the two Pasteurian vaccines and from that it was wished to 

 conclude that these vaccines should not be employed in practice to 

 protect sheep against the anthrax fever. The results of experiments 

 made on a large scale in various parts of the globe have demonstrated 

 the inadequacy of these objections and these questions are now 

 regarded as definitely settled. 



So large a number of investigations, in response to the most urgent 

 and immediate needs, was not favourable to minute researches on the 

 mechanism of this immunity which had been revealed in so marvellous 

 a fashion. In spite of this, Pasteur applied himself to the solution 

 of this problem so far as this was possible under the conditions in 

 which he carried on his investigations. He thought that acquired 

 immunity was the result of the impossibility of the growth of a 

 pathogenic micro-organism in a medium in which it had previously 

 been cultivated. When the micro-organism of fowl cholera sets up in 

 certain individuals a disease which though grave is not fatal, or 

 when the attenuated micro-organism produces a simple, transient 

 discomfort, it lives in both cases in the fluids and tissues of the 

 animal. This existence is possible in consequence of the absorption 

 of certain nutrient substances. Once these substances are consumed 

 they are not easily renewed, and in consequence the vaccinated 

 organism becomes incapable of nourishing the special micro-organism 

 a second or a third time. To support this brilliant hypothesis by 

 precise facts Pasteur made experiments on the conditions met with 

 in the development of the micro-organism of fowl cholera in vitro. 

 He filtered a broth culture of this micro-organism after it had grown 

 luxuriantly for several days, and into the fluid, which had now 

 become clear and transparent, he sowed afresh the same micro- 

 organism. No growth took place and the fluid remained quite clear. 

 This absence of development might be explained either by the 

 presence in the fluid of some excremental substance thrown off 

 during the first culture or by the absence of some substance indis- 

 pensable for the nutrition of the micro-organism. Pasteur excluded 

 the first hypothesis by an experiment which demonstrated that it 

 is sufficient to add to the filtered fluid a small quantity of fresh 

 nutritive substances to enable the micro-organism again to develop 

 abundantly. It is therefore to the absence of some element essential 

 to the existence of the micro-organism that we must attribute the 

 immunity enjoyed by animals which have been vaccinated or which 



