CHEMOTAXIS 



23 



most diverse chemical constitution. Many compounds were 

 found to exercise a repellent effect, especially in strong con- 

 centration. Amoeba reacts negatively to most chemicals 

 that affect it. Where it comes in contact with the diffusing 

 chemical the ectoplasm contracts; currents beginning in the 

 stimulated region gradually extend through the body; 

 pseudopods are thrust out opposite the stimulated part 

 and the Amoeba begins to crawl away. Positive chemo- 

 taxis probably plays a part in the food taking of Amoeba, 

 although its role has not been clearly worked out. In the 



FIG. 1. A. B. C. Sucessive phases of the reaction of Amoeba to a 

 chemical (represented by the dotted area). 



amoeboid plasmodia of myxomycetes, which usually live upon 

 decaying wood, Stahl has found that extract of bark pro- 

 duces a positive response. 



Chemotactic reactions are as a rule pronounced among 

 the infusoria. Paramoecia and some other forms collect in 

 weak acids, while they are generally negative to stronger acids 

 and to alkalies. If a drop of strong carbon dioxide solution 

 is placed in the midst of a lot of Paramoecia the infusorians 

 will form a ring about it. Being negative to the stronger 

 acid at the center of the drop and positive to the weaker 

 concentration that surrounds it they naturally collect in a 



