OBSERVATIONS ON UNICELLULAR FORMS 



79 



exposed the direction of motion was nearly reversed. This 

 indicates that the reaction was modified, that the response 

 to a given stimulus depends upon the preceding experience. 



Fig. g. Sketches representing the reactions of an amoeba proceeding toward 

 an intense area of light the rays of which were perpendicular to the slide. L, field 

 of light formed by focusing a section of a Welsbach mantle on the slide, i-io, 

 successive positions of the amoeba a little less than one-half minute apart. Arrows 

 indicate direction of streaming in pseudopods. 



In view of these facts it is probably true that the orienta- 

 tion of all of the rhizopods in light is due to a local response 

 to a local stimulation, a direct inhibition of the movement 

 of the part most highly illuminated. This would of course 

 result in the prevention of the formation of pseudopods on 

 the more highly illuminated side, and the organism would 



