REACTIONS OF AMEBA TO LIGHT. 6 1 



Another proteus was then brought into the field with the dark 

 spot slightly to the left 354. The tip of the main pseudopod 

 broke up into two pseudopods of which one moved directly toward 

 and over the dark area. But when the ameba came into contact 

 with the dark spot, a large pseudopod was thrown out on the 

 right 356 but it was withdrawn as the ameba moved on over 

 the dark spot. The ameba reacted in effect positively throughout 

 the experiment, but a strong tendency to react negatively is 

 shown by the breaking up of the main pseudopod into two 

 pseudopods 355 and by the appearance of the pseudopod on 

 the right when the ameba came into contact with the dark spot 

 358. The ameba was then shifted with the dark spot slightly 

 to the left 359. The ameba moved forward a short distance, 

 then the tip of the main pseudopod spread out, and then the 

 protoplasmic stream was suddenly reversed and the ameba 

 moved away to the right through a vestige of a previous pseu- 

 dopod a decided negative reaction. But the ameba was then 

 moved with the dark spot directly ahead 363. After the ameba 

 had moved forward a short distance the tip forked broadly, and 

 the ameba moved off through the left prong, again a decided 

 negative reaction. 



Summary. Amebas become aware of dark spots before they 

 come into contact with them, as seen through the microscope 

 with the eye, just as they become aware of beams of light before 

 encountering them. In most cases the tendency is to react 

 negatively, but in some instances the first change in behavior is 

 positive. Usually when the ameba first comes into contact with 

 the dark beam there is a tendency toward negative behavior, as 

 is shown by the formation of pseudopods which, if they became 

 main pseudopods, would lead away from the dark area. These 

 pseudopods are usually withdrawn as the ameba moves forward 

 over the dark spot. The behavior is seldom wholly positive or 

 wholly negative; in most cases there is some vacillation between 

 negative and positive reactions. The reactions on the whole 

 were not so pronounced as those toward light. The actual 

 stimulating quality is very likely to be looked for in the difference 

 in light intensity between the dark spot and the field. 



