OBSERVATIONS ON UNICELLULAR FORMS III 



by changing the direction of the rays or by changing the 

 light intensity of the field. 



(7) The bending reactions are induced wherever the light 

 strikes the dorsal side of Euglena owing to its rotation on 

 the long axis. This reaction is repeated until the organism 

 is oriented and rotation no longer causes a change of illumi- 

 nation on its dorsal and ventral surfaces. It remains ori- 

 ented because, while it proceeds in this direction, there are 

 no stimulations which induce the bending reaction. 



(8) The intensity can be so gradually changed that there 

 is no response. The bending reaction is therefore depend- 

 ent upon a time rate of change, and orientation is conse- 

 quently also due to a time rate of change in the light 

 intensity. 



(9) The results of these experiments support Jennings' 

 conclusion that orientation in Euglena is brought about by 

 selection from trial positions. This of course does not 

 mean conscious selection. 



(10) It is probable that light has a constant effect on the 

 activity of Euglena much as temperature does, but there is 

 no evidence that such activity has anything to do with the 

 process of orientation, as the explanations of Loeb, Ver- 

 worn, and Torrey demand. 



(11) Orientation is not dependent upon the direction in 

 which light rays pass through the tissue, in accordance 

 with Sachs; nor is it dependent upon the angle between 

 the rays and the sensitive surface, or the unequal stimula- 

 tion of symmetrically located points on the surface, as Loeb 

 assumes; nor upon the effect of the stimulating agent upon 

 the locomotor organs directly, or through a direct reflex 

 arc, as the theory of Verworn demands; nor upon light 

 acting constantly as a directive stimulus, in accord with 

 Loeb's idea supported by Torrey. 



(12) The most highly sensitive portion of Euglena is 

 probably situated near the concave surface of the eye-spot, 

 and the eye-spot probably functions in casting a shadow 

 on the highly sensitive substance when the light strikes the 



