RESPONSES BY AMCEBA. 411 



The refractory period used as a basis for the experiments pre- 

 sented in this paper, during which recovery from the effects of 

 previous stimulation occurs, has long been known, especially when 

 light has been used as the stimulating agent. In this case the 

 term dark adaptation has been used. An animal must be dark- 

 adapted before it will respond to illumination. This necessity for 

 dark-adaptation has led to an explanation of the response to light 

 which involves the presence in the organism of a substance cap- 

 able of a reversible photochemical reaction (Mast, '07; Hecht, 

 '18). The reaction is thought of as being initiated by the con- 

 version of a photosensitive substance into its precursors, and dark- 

 adaptation as the reforming of this substance from the precursors. 



As we have just seen, however, recovery from the effects of a 

 mechanical shock may be necessary before a response to sudden 

 illumination is possible, and it would therefore seem that the same 

 processes are involved in the refractory periods which follow the 

 two types of stimuli. If so, since a photochemical reaction can- 

 not occur in the case of mechanical shock, it is evident that in 

 Amoeba, at least, dark-adaptation must consist of something be- 

 sides the reforming of a photosensitive substance from its pre- 

 cursors. 



SUMMARY. 



1. Amceba responds both to mechanical shock and to sudden 

 illumination by a cessation of movement, which does not take 

 place immediately on stimulation, but after a considerable reaction- 

 time. 



2. In both cases the reaction-time depends upon the magnitude 

 of the stimulus, becoming longer as the intensity of the stimulat- 

 ing agent increases. 



3. In both instances the time during which the amoeba is in- 

 active also depends upon the magnitude of the stimulus, becoming 

 longer as the latter increases. 



4. After an amoeba has been exposed to light it is necessary 

 that a certain interval of time elapse before it will again respond 

 to sudden illumination. Likewise, after a mechanical shock the 

 amoeba must be allowed time for recovery before it will respond 

 to a second shock. 



