184 S. O. MAST 



ceived; i.e., in case of light, it depends upon the intensity of the 

 illumination. 



Shock reactions unquestionably function in the process of 

 orientation in many of the lower forms, e.g., Euglena, Stentor, 

 Volvox, blow-fly larvae, etc. (Mast, '11). Bancroft ('13), in a 

 valuable paper on Euglena, agrees with this contention, but he 

 also maintains that there are responses dependent upon the 

 continuous action of light, which function in the process of 

 orientation. I have elsewhere ('14) presented evidence indicat- 

 ing that Bancroft's conclusion in reference to the continuous ac- 

 tion of light is questionable. However this may be, the orienting 

 reactions in the tadpoles of Amaroucium are largely (probably 

 entirely) of the shock, trigger, or ' all-or-none ' type, those in- 

 duced by an increase as well as those induced by a decrease 

 in the illumination of the retina. They are dependent upon 

 the time-rate of change in illumination and there is clearly a 

 threshold. If this is exceeded, there is a reaction. Those reac- 

 tions which are induced by decrease in illumination are of a 

 given magnitude which is not increased no matter how great the 

 reduction in illumination beyond the threshold may be. And 

 this probably holds also for those which are induced by increase 

 in illumination, although in these the magnitude of the reaction 

 may possibly depend somewhat upon the magnitude of the 

 stimulus. If there is actually such a relation it can, however, 

 i:eadily be accounted for by the assumption that more muscle 

 fibers come into play when the stimulus is strong than when it is 

 weak, just as is done in applying the ' all-or-none ' law to cases 

 in which such relations are known to obtain. This matter is 

 of considerable importance and it should be more thoroughly 

 investigated. 



