Vol. XLVI. February, 1924. Xo. 2. 



BIOLOGICAL BULLETIN 



MODIFICATION OF RESPONSE IN AMU.I'. V 



S. O. MAST AND L. C. PUSCH. 

 FROM THE ZOOLOGICAL LABORATORY OF THE JOHNS HOPKINS VMVI .K-IIV. 



Gibbs and Dellinger ('08) and Schaeffer ('17) maintain that 

 AiiKJL-ba has the power of selecting food. The former hold that 

 the kind of food preferred changes under various condition-, .unl 

 they suggest that changes in the kind of food selected is the re- 

 sult of a process of learning based upon the method of "trial and 

 error." One of us (Mast '10) in preceding observations found 

 that when the tip of a pseudopod of an amoeba comes in contact 

 with a highly illuminated region in the field it stops; then others 

 are sent out in succession in the same general direction, each one 

 stopping when it comes in contact with the light, until suddenly 

 one is sent out in a markedly different direction, avoiding the 

 illuminated region altogether. It is maintained that this abrupt 

 change in tin- direction of the formation of the pseudopods. 

 constituting a profound modification in the response to a given 

 stimulus, is dependent upon the preceding experience of the 

 individual, i.e. repeated contact with the strong light. 



Tin- specimens used in the following experiments were obtained 

 from the cultures in the laboratory, most of which had been 

 -torked \\ith material from a stream near the campus. They 

 were of the proteus type. Each amoeba was isol.ited on a glass 

 slide in a drop of its culture-fluid under a cover-.^l.i . tin ed^es of 

 which were sealed with vaseline. Each anm-li.i was adapted to 

 darkness before using. 



The observations were made in tin- d.irk room. An area of 

 intense light was obtained by forii-iiii; on the >lidc under the 

 microscope, by means of the reflector .md ,m Abbe condenser, 

 an image of a portion of the luniim >us til.unent < >l .111 incandescent 

 Ma/da lamp placed in a light-tight 1>\. I lie IM\ - if the filament 

 -ed through a very narrow slit in the box. The image h.nl 



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