AMEBOID MOVEMENT 15 



formation is a racial characteristic and is hereditary. The 

 streaming endoplasm therefore, upon suitable stimulation, takes 

 on a definite form, that of a food cup. This indicates again that 

 the endoplasm is something more than the ordinary fluids of 

 physics, for out of an apparently structureless fluid, organization 

 is effected. 



The fact that food cups are formed by amebas implies of course 

 that stimuli are received whose effect cannot be explained as a 

 direct physical reaction. Rhumbler ('10) has attempted to ex- 

 plain the formation of food cups as the direct physical result of 

 the stimulation by the food body ; but in recent experiments 

 Schaeffer ('16) has shown that food cups are formed over dif- 

 fusing solutions of tyrosin, where the solutions were quite as 

 concentrated outside as inside the cup. These results prove con- 

 vincingly that the shape and size of the food cup are not deter- 

 mined by direct action of the stimulating agent, but by hereditary 

 factors within the protoplasm of the ameba. 



Other stimuli also affect streaming characteristically, though 

 not so strikingly perhaps as food stimuli. One of the most widely 

 observed effects on streaming is the momentary pause following 

 stimulation of many sorts. If an ameba that is moving along 

 unstimulated externally, suddenly comes near a food object, it 

 frequently stops forward streaming for about a second, and then 

 begins again, usually at . increased speed. The ameba behaves 

 as if it were startled. A similar reaction is observed if a small 

 perpendicular beam of light is flashed near the anterior end of 

 the ameba. Here also streaming is resumed with accelerated speed 

 toward the beam of light. Harrington and Learning ('oo) showed 

 that if strong light, especially at the blue end of the spectrum, 

 is suddenly thrown on the ameba, movement is arrested for a 

 short time. Miss Hyman ('17) has shown recently that if an 

 ameba is strongly stimulated with a glass needle, streaming is 

 arrested momentarily, but the direction of streaming when re- 

 sumed subsequently, depends partly upon the former direction of 

 streaming and partly upon the location of the stimulus. All of 

 these cases of temporarily arrested movement are strikingly sim- 

 ilar to what is observed in the higher animals under similar con- 

 ditions. 



