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 Amceba begins to crawl away. Positive chemo- 
taxis probably plays a part in the food taking of Ameba, 
although its réle has not been clearly worked out. In the 
A 
Fia. 1.—A. B. C. Sucessive phases of the reaction of Ameba 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. Paramcecia 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 Parameecia 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 
