lOO BIOLOGICAL LECTURES. 



first brought upon the slide, almost every individual in the prep- 

 aration will in a short time have come by chance against the 

 edge of the drop, will have entered and remained, so that soon 

 all the Paramecia in the preparation are in the drop. 



Thus it appears that the animals are not attracted by the 

 fluid in the drop ; they enter it by chance, without reaction, 

 then are repelled by the surrounding fluid. The peculiar fact 

 that the animals, after entering the drop of the substance in 

 question, are repelled by the surrounding fluid in which they 

 were previously immersed will become more comprehensible 

 after the phenomena of repulsion are considered. 



Turning, then, to the matter of negative chemotaxis or 

 repulsion, we have the following phenomenon to be explained. 

 If into a slide of Paramecia swimming at random a drop of some 

 repellent chemical (as NaCl) is introduced, we find that the 

 drop remains entirely empty, not a single Paramecium entering 

 it. Now, exactly how do the Paramecia succeed in keeping out 

 of such a repellent solution } 



Careful observation shows that when the Paramecium, swim- 

 ming forward, comes in contact with the drop of repellent sub- 

 stance, it swims backward a short distance, then turns toivard 

 its own ahoral side, then swims forward again. The essential 

 point in this reaction method is, that the Paramecia always turn 

 toward their own aboral side, without regard to the position of 

 the stimulating drop. If a Paramecium comes obliquely in 

 contact with the drop so as to touch it only on one side of its 

 body, it nevertheless gives the reaction above described with- 

 out modification, even though turning toward its own aboral 

 side after backing off may carry the animal directly toward the 

 drop, instead of away from it. In such a case the animal when 

 it comes again in contact with the drop simply repeats the 

 reaction. As it continually revolves on its long axis both when 

 swimming forward and Vhen swimming backward, the aboral 

 side is nearly certain to lie in a new position the second time, 

 so that the animal turns in a new direction. If this is repeated 

 a sufficient number of times, the Paramecium is fairly certain, 

 • by the laws of chance, to get started finally in a direction which 

 carries it away from the stimulating chemical. 



