THE EFFECT OF EXTERNAL INFLUENCES ON CELLS 55 



these may be regarded as to a certain extent chemotactic. Thus according to 

 the thorough analysis of Bethe, a whole order of complicated habits of the 

 ants might be explained as chemotactic reactions, and in the bees several habits 

 are undoubtedly of this origin. 



Jennings has shown that there is nothing specifically directive about the 

 chemotactic effects of chemical substances on Infusoria and Bacteria. If, for 

 example, Bacillum volutans be placed in a preparation with a green alga, they 

 are uniformly distributed at first throughout the preparation. When the alga be- 

 gins to give off oxygen and the bacilli come by chance into this zone rich in O 2 , 

 they sw r im through it to the opposite side, turn and swim again to the border, 

 and so on incessantly, but they do not adhere to any definite orientation with 

 respect to the middle point of the oxygen zone. 



[The behavior of an Infusorian under chemical stimulation may be illus- 

 trated, according to Jennings, as follows : the usual motor response of a Paramce- 

 cium to any kind of an obstruction is to reverse its cilia and swim backward, 

 then turn toward the side containing the peristome and swim forward again 

 (Fig. 32). When in its wanderings the Paramwcium enters a drop of dilute acid, 

 the chemical change of the medium does not cause the reaction, but whenever 

 the organism attempts to leave the drop the chemical change experienced consti- 

 tutes a stimulus which evokes the usual motor response, and the organism remains 

 entrapped. Coming in contact with an alkali produces the same response and 

 the organism turns so as to avoid the substance. The result is that the organ- 

 isms collect in dilute acids, including carbon dioxide (positive chemotaxis), and 

 refuse to do so in alkalies (negative chemotaxis). But the acid can scarcely be 





FIG. 32. Motor response in Paramoecium, after Jennings, a-/, successive positions after meet- 

 ing an obstruction, A. 



said to attract the organisms in any proper sense of the term, nor the alkali to 

 repel them. They remain in the acids doubtless because these substances are 

 favorable l to their life processes and avoid the alkalies because the latter are 

 harmful. ED.] 



1 Jennings recognizes this selection by random movements of conditions not interfering 

 with the physiological processes as a fixed principle of behavior, not only in the Bacteria 

 and Infusoria but in higher animals as well. ED. 



