THE MOVEMENTS AND REACTIONS OF AMCEBA. 187 



Much more striking cases of the regulation of the movement in 

 accordance with the position and changes in position of outward things 

 ( u automatic acts," Ziehen, /. c.) than are found in such a reaction, or 

 even than in the possibility of driving an Amoeba, will be described 

 in the account of the food reactions (p. 196). 



REACTIONS TO CHEMICAL STIMULI. 



By analogy with the effects of mechanical stimuli we might expect 

 to find a positive reaction to chemical stimuli. Such reactions doubt- 

 less occur, but I have not been able to demonstrate them under experi- 

 mental conditions, and, so far as I am aware, no one else has succeeded 

 in doing this. Stahl (1884) has shown the corresponding reaction to 

 take place in the myxomycete plasmodium. Verworn (1890, p. 456) 

 records an observation which he refers with much probability to a 

 positive reaction to a chemical stimulus in Difflugia. If two conjuga- 

 ting Difflugias were separated, they crept directly together again, and 

 it is difficult to see how the movements could have been directed save 

 by some chemical. But I believe there is no instance of positive chemo- 

 taxis in Rhizopoda where the nature of the active substance is known 

 and the reaction was controlled experimentally. A number of striking 

 positive reactions, which should probably be attributed partly to chemi- 

 cal stimuli, are described later in connection with the attempts of 

 Amoeba to obtain food (p. 196). 



The same state of affairs has existed hitherto with regard to our 

 knowledge of a negative reaction to chemicals. In Amoeba it is, how- 

 ever, not difficult to produce such reactions experimentally. 



For this purpose only a small amount of the chemical must be used, 

 so that it can act on but a limited portion of the body of the animal. 

 If there is a considerable amount of the solution, diffusing over a large 

 area, it reaches a strength sufficient to cause a reaction at about the same 

 time over the whole body of the Amoeba ; thus the reaction is a general 

 one, not involving movement in a definite direction. To produce a 

 directed reaction there must be a decided difference in the strength of 

 the solution on two sides of the organism. 



The easiest method of producing the reaction, and the one giving at 

 the same time the most striking results, is to dip the moistened tip of 

 a capillary glass rod into powdered methyl green or methyline blue ; 

 then to bring this near one side or end of the Amoeba, in an open drop 

 of water. The chemical diffuses in a colored cloud ; the reaction takes 

 place when the edge of this cloud comes in contact with the Amoeba. 



The reaction is essentially the same as that to mechanical stimuli. 

 The region stimulated stops suddenly, and about a second later con- 

 tracts, while a current moves away from the side stimulated. This 

 may meet the previously existing current coming from the original 



