32 CHEMICAL AGENTS AND PROTOPLASM [Cn. I 



of acclimatization to ricin. Mice which had gained an immu- 

 nity of over 200, and were then kept for 6.5 months on normal 

 food, had still a resistance, although not precisely determined, 

 certainly far above 50. 



It is an important question : Is an organism acclimated to 

 one poison thereby rendered more resistant to poisons in gen- 

 eral, or only to the specific poison to which it has been accli- 

 mated? EHRLICH found that mice acclimated to ricin were 

 just as sensitive to abrin as the normal animals, and the same 

 is true, mutatis mutandis, for mice which resist abrin. 



Concerning the changes in the protoplasm brought about by 

 acclimatization little is known. 



EHRLICH ('91) and CALMETTE ('94) have shown that in the 

 blood of the immunized animal a substance, antitoxic to the 

 specific substance employed, is produced, and this apparently 

 prevents the action of the strong poison by transforming its 

 molecules. The antitoxic substance is of such a nature that 

 when blood containing it (from an acclimatized animal) is 

 injected into an unacclimatized one, the latter becomes immune 

 to the poison. 



For Protista another hypothesis is admissible ; namely, that 

 the weak solution of the poison, which is used in acclimatiza- 

 tion, gradually destroys those compounds upon which the 

 strong solution would have acted suddenly and, therefore, 

 fatally. The gradual destruction is not fatal because of its 

 slowness. At the same time it prevents the violent action of 

 the strong poison, since it leaves it nothing to be acted upon. 



The parallelism between the results of experiments upon 

 acclimatization to poisons and those upon immunization through 

 vaccination, leads to the suspicion that, at bottom, the two 

 processes are closely akin. 



3. CHEMOTAXIS 



ENGELMANN ('81) seems to have been the first to show that 

 the direction of locomotion of simple protoplasmic masses is de- 

 terminable by chemical agents in the environment. He found 

 that Bacterium termo is thus acted upon by oxygen which is 

 not uni'formly distributed. Like many Infusoria, these bacteria 



