CHEMOTAXIS 27 



tion of the body, no seeking of the stimulus and no attraction 

 in any sense. The Paramoecia as they happen to approach. 

 a drop of dilute chemical to which they are positi 

 chemotactic, swim into it without interruption, but when 

 they encounter the other side of the drop and experience 

 the transition from weak acid to water they give the motor 

 reflex or avoiding reaction and change their course. As 

 often as the Paramoecium encounters this boundary it 

 reacts in the same way. It behaves as if caught in a trap 

 which presents no obstacle to its entrance, but effectually 

 prevents its exit. Other Paramoecia swimming by chance 

 into the drop are also caught there and soon a collection is 

 formed which may ultimately include all the individuals of 

 the region. In the negative response the method is much 

 the same only it is the transition from weak to strong acid 

 which causes the stimulation. The avoiding reaction is 

 given in the same way hi each case. All of the chemotactic 

 responses take place without any orientation of the body, 

 by a method of trial and error. 



In the flagellate Chilomonas Jennings has shown that 

 chemotactic grouping takes place just as in Paramcecium. 

 In the bacterium Spirillum he finds much the same method 

 employed. Spirillum swims by the movement of flagella 

 at one or both ends of the body. When stimulated it 

 simply reverses its direction of movement. Positive and 

 negative reactions take place in exactly the same way. 

 What things are avoided and what are sought depends 

 upon what acts as a stimulus causing the reversing reaction. 

 Apparently these simple organisms have no power of 

 orienting their bodies; they simply move back and forth, 

 relying on chance to get them into a favorable situation. 

 When such a situation has been reached the organisms 

 remain relatively quiet. In several cases among the fiagel- 



