IMMUNITY. . 311 



must be present in these products which act 

 unlike the poisons. 



To put it briefly, the protecting bodies are 

 either the bacterial cells themselves, or they 

 exist in the cell-body, or they become separated 

 externally from the cells. In the so-called 

 metabolic products present in culture fluids 

 after bacterial growth are found not only the 

 true toxins but always along with these the 

 dead bodies of the bacteria, together with the 

 dissolved portions of dead bacteria and the sol- 

 uble cell substances that have separated from 

 the bacteria during life. In the active culture 

 fluids, not only the poisons, but cell substances 

 of another kind are present in solution, and 

 both of these may be separated by filtration 

 from the living or dead bacterial cells. If cul- 

 ture fluids in which active parasites have grown 

 be heated to about 70 the " specific " poisons 

 are destroyed or changed, although at this tem- 

 perature the soluble cell substances which 

 confer immunity appear to suffer no essential 

 alteration. But not only those parasites which 

 are attenuated and form little poison and those 

 that have become impotent and no longer form 

 poison, but also certain harmless saprophytes 

 which form no poison at all, possess such 

 soluble protective cell substances. It is very 

 improbable that the protecting substances are 



