THEORIES OF IMMUNITY. 



tube; for if a fluid in which a micro-organism 

 has grown be filtered and planted with a fresh 

 culture, no growth will occur. This certainly 

 might be due to the exhaustion of the nutrient 

 medium, and that it is so seems to be shown by 

 the fact that an addition of a very small amount 

 of fresh medium is followed by a profuse growth 

 of the bacterium in question. The same thing 

 was supposed to occur in the living tissues. 



Such a theory, however, would not account for 

 the facts seen in Natural Immunity, in which no 

 previous growth of the bacteria could be sup- 

 posed to occur. Nor would it explain the results 

 seen in Algerian sheep. These animals are not 

 susceptible to inoculation with anthrax bacilli 

 sufficient to kill ordinary French sheep, but will 

 succumb to very large doses of such bacilli. If, 

 then, the first resistance was due to the absence 

 of nutrition for a comparatively small number 

 of bacilli, how could a much larger number find 

 enough to live upon? 



In view of such objections as these, Chauveau 

 brought forward his Retention Theory to explain 

 the phenomena seen. In accordance with this, 



