414 FRANK W. BANCROFT 



trast between heliotropic and galvanotropic orientation will not 

 hold for this organism. 



There remains to be made out the pole of the organism at which 

 the current stimulates, and the location of the sensitive region, 

 the stimulation of which brings about this reaction. The sensi- 

 tive region is probably in the vicinity of the base of the flagellum. 

 If the current is made when the Euglena has its anterior end 

 towards the cathode very little if any change is produced. If, 

 however, it is made when the anterior end is towards the anode 

 there is intense stimulation (fig. 5, c and d). The flagellum ex- 

 tends forwards, the Euglena swims backwards a little and then 

 contracts. Thus there can be no doubt that in this case the 

 stimulation is at the anode just as it is in Paramecium that has 

 been subjected to citrates for a short time (Bancroft, '06). 



Consequently as the stimulation in the transverse Euglena takes 

 place when the dorsal side is towards the cathode, we must sup- 

 pose that at this moment the spot where stimulation occurs is 

 anodal with respect to that part of the body from which it receives 

 its stimulating ions. In this way we see that, given the normal 

 locomotor mechanism, all that is necessary to make cathodal 

 galvanotropism possible is the presence of some substance which 

 is affected by certain anions in such a way as to increase the turn- 

 ing effect of the beat of the flagellum. But this substance must 

 be so located that it is anodal to the stimulating anions when the 

 dorsal side of the Euglena is turned towards the cathode. 



I. Conclusions 



The facts of the light reactions of Euglena described by Jen- 

 nings and Mast have been confirmed in all cases in which they were 

 reinvestigated. No differences of opinion exist as regards these 

 facts. But in interpreting the facts Jennings and Mast have ex- 

 plained them by means of a theory which does give a formal ex- 

 planation but is supported by no evidence beyond the facts which 

 it was intended to explain ; and which, as we have seen, is not capa- 

 ble of standing the strain of explaining the additional facts brought 

 forward by experiments especially devised to test the correctness 

 of the theory. 



