AMEBOID MOVEMENT 121 



Figure 41. Showing the phenomenon of encircling, after Schaeffer. The 

 ameba moved around a perpendicular beam of red light. The reaction 

 was neither distinctly positive nor negative. 



wave, instead of running into the base of the next wave is re- 

 flected backwards to form a circular curve. All the evidence 

 thus indicates that the weakest point is at the base of the wave. 

 A constantly acting stimulus may therefore break the wave here 

 if it cannot break into it elsewhere, and so change the direction 

 of the path. In Figure 42 are shown a few diagramatic waves 

 in the path of an ameba together with several reflected curves at 

 i, 2 and 3 indicating the points at which the direction is most 

 easily changed as evidenced by the temperature experiments. 

 If a particle within sensing range of the ameba lie at a, b, or c, 

 and stimulate the ameba only slightly but still enough to break up 

 the wave formation, the ameba will take a curved path around 

 the particle as indicated by the dotted lines. But if the same 

 particle lay at any other point with reference to the position of 

 the wave, as at e, f, or g, the ameba would not have changed its 

 course. Briefly, the following conditions must be satisfied to en- 

 able the phenomenon of encircling to appear: (i) The particle 

 must lie a little to the side of the ameba's path. (2) It must lie 

 abreast of the point at which the ameba begins to change its direc- 

 tion of movement (i. e., at the base of the wave) when describing 



