CHOICE OF FOOD IN AMEBA 229 



not completed — 197. The needle at no time touched the 

 ameba. This experiment, in connection with the preceding 

 ones, shows that a purely mechanical stimulus is sufficient to 

 set in operation the ameba 1 s feeding mechanism. 



The effect of vibrating insoluble indigestible particles: Glass. — 

 A clean piece of glass was laid near an A. dubia and slightly 

 agitated with a clean glass needle — 226. The glass particle was 

 promptly ingested in a typical food cup with a large amount of 

 water. The ameba did not undergo a period of rest but kept 

 on moving forward and in six minutes the glass was excreted — 

 232. It had been completely surrounded by protoplasm. Five 

 minutes later the same piece of glass was again agitated with 

 a glass needle — 234. Again the glass was ingested in a typical 

 food cup. Three and one-half minutes later the glass was 

 excreted — 237. Another trial with the same piece of glass pro- 

 voked the formation of a food cup until it was about three- 

 fourths completed, then it was retracted — 238. Two further 

 trials produced visible responses, but no complete food cup was 

 formed. The ameba became increasingly indifferent toward the 

 artificial stimulation with each succeeding trial. 



Silicic acid. — An A. dubia was stimulated with oscillatoria 

 threads until a food cup was partly formed, then a grain of 

 silicic acid was placed in it and stimulation with the alga fila- 

 ments resumed — 240. The food cup closed up apparently com- 

 pletely, for three minutes after the closing of the cup the silicic 

 acid was actively thrust out as a piece of undigested food material 

 might have been thrown out — 244. 



Carbon. — A fragment of purified carbon was laid on the back 

 of a raptorial ameba — 245. The carbon was raised up by the 

 protoplasm immediately underneath it, but very soon there- 

 after it rolled down the side of the ameba. A large pseudopod 

 was then sent out over the carbon but no distinctive food cup 

 was formed. When the ameba was forcibly rolled over, it was 

 found that the carbon adhered to the ameba. No attempt had 

 been made to surround it after the psuedopod was laid over it. 



Indigotin. — A piece of indigotin was laid in front of an A. 

 dubia and agitated with a glass needle — 247. The indigotin 

 was eaten by means of a typical food cup containing a large 

 quantity of water. The ameba moved on in the original direc- 

 tion. Nine minutes after the indigotin was ingested, it was 



