'heliotropism in euglena 389 



Jennings in my disapproval of it. For while he thinks that it 

 holds in galvanotropism, I think it more probable that the catho- 

 dal reversal of the cilia in Paramecium is not brought about by 

 the action of the current directly on the cilia, but on some inter- 

 mediate mechanism. I do not wish to deny, however, that there 

 may be some reactions such as those of Ameba mentioned by 

 Jennings ('06, p. 306) for which the local action theory may hold. 



B. Position of Jennings and Mast on direct orientation 



Before proceding to the experimental data it will be best to 

 outline Jennings's and Mast's position so that the significance of 

 my results may be correctly understood. 



In his earlier papers Jennings found that most infusoria when 

 encountering an obstacle react by what he called a motor reflex. 

 The essential element of this reflex is the turning towards a struc- 

 turally defined side of the organism. The direction of turning 

 bore no relation to the part of the organism stimulated or the 

 location of the source of the stimulus. The stronger the stimulus 

 the greater the amount of turning; but the exact point at which 

 ' the turning stopped and the usual forward locomotion began bore 

 no relation to the location of the source of stimulation. Often 

 the infusorian would be brought repeatedly in contact with the 

 stimulus and repeated motor reflexes would result. In such a 

 reaction the final direction of progression may be said to be due 

 to the selection of random movements; for the direction of turning 

 and the final direction of progression are with respect to the posi- 

 tion of the stimulus determined only by chance. In subsequent 

 papers this type of reaction was called by Jennings 'trial and 

 error.' 



Later Jennings found ('04, p. 31) that Stentor when subjected 

 to an increase in illumination gave the typical motor reflex and 

 that it also became oriented in a uniformly illuminated field by 

 means of a series of motor reflexes. The turning was always to- 

 wards a structurally defined side, and after each reflex the animals 

 found themselves in the most various positions. But if the light 

 was shining on their anterior ends the reflex was always repeated. 



