STIMULI AND THEIR ACTIONS 



439 



not sufficient that a certain quantity of the substance in question 

 be uniformly distributed in the vicinity of the organism, but a 



Z.---&: 



FIG. 215. Chematoxis of Paramcecium aurelia. A, Chemotactic cover-glass preparation ; a drop 

 of liquid that induces negatively chemotactic properties has been introduced under the cover- 

 glass by means of a capillary pipette. , Positively chemotactic assemblage. C, The same 

 with too strong a solution ; the Paramacia have congregated in a circle at the optimum-point 

 of concentration. D, A bubble of carbonic acid and one of air are under the cover-glass ; the 

 former (at the left) induces positively chemotactic properties ; the latter is indifferent. , 

 The same preparation a few minutes later ; the carbonic acid has diffused into the surrounding 

 water and by its too high concentration has driven the Paramcecia to the place where they 

 find their optimum of carbonic acid. (After Jennings.) 



decrease in concentration must take place on one side. In other 

 words, it is the amount of the difference in concentration at the 



