CHOICE OF FOOD IN AMEBA 227 



The effect of vibrating food particles. — A small fragment of an 

 ameba was placed in the path of an A. proteus. Pseudopods 

 were sent out toward the fragment, but all of them were soon 

 withdrawn — 198-205. The fragment was then allowed to roll 

 down the side of the ameba, whereupon ingestion followed at 

 once — 206. It is certain that the mechanical stimulation pro- 

 duced by the rolling fragment hastened ingestion. 



An A. dubia was then tested ten times with grains of globulin, 

 but only one was eaten, the seventh. On the eleventh trial a 

 piece of globulin was laid against a newly formed pseudopod 

 and agitated with the point of a glass needle. A food cup was 

 promptly formed and the globulin apparently ingested — 207. 

 Five minutes later the ameba moved on, leaving the globulin 

 behind in about the same place — 210. It had not been com- 

 pletely surrounded by protoplasm. The formation of the food 

 cup in this case was caused by the vibration of the globulin. 



Another A. dubia that reacted rather indifferently toward 

 globulin, although the globulin was finally eaten, and quite 

 indifferently toward ovalbumin, iron and fibrin, was tested with 

 a grain of fibrin agitated with a glass needle. A food cup was 

 formed and the fibrin apparently ingested at once in a food 

 cup containing a large quantity of water — 211. The ameba 

 became quiet for a minute and a half and then moved off in 

 the original direction, leaving the fibrih behind, four minutes 

 after the formation of the food cup — 212-216. The fibrin was 

 probably not completely surrounded by protoplasm. The for- 

 mation of the food cup was caused by the vibrations of the 

 fibrin. 



The effect of vibrating alga filaments. — One of the most illu- 

 minating experiments bearing on the general problems of mechan- 

 ical stimulation is the following: A raptorial ameba which had 

 reacted indifferently to capillary tubes filled with peptone solu- 

 tion was gently stimulated mechanically by the free ends of 

 three thin oscillatoria threads wrapped around a glass needle. 

 Stimulation at the posterior end produced a prompt reversal 

 of streaming. When stimulated at the newly formed posterior 

 end, streaming was again reversed. In this way the direction 

 of the protoplasmic current was reversed eight times in suc- 

 cession, the current always moving toward the point of stimu- 

 lation, without bringing the ameba away from its original location. 

 Figure 217 shows the position of the ameba while the proto- 



