OBSERVATIONS ON UNICELLULAR FORMS 95 



necessary to induce a reaction will be discussed later 

 (p. 105). 



If the intensity from the two sources of light arranged as 

 described above is equal and the beams which reach the 

 stage of the microscope are alternately cut off with an 

 opaque screen so as to change the direction of the rays 

 suddenly without changing the intensity, it appears as 

 though the Euglenae if positive always turn directly toward 

 the source of light, never away from it no matter in what 

 position they are or which surface becomes illuminated 

 when the ray direction is changed. These results would 

 seem to indicate that there is here a local response to a 

 local stimulation, or at least differential response to local- 

 ized stimulation. I was firmly convinced of the truth of 

 this for several days, as were also other members of the 

 laboratory who observed these reactions. Further work 

 however demonstrated the fallacy of this conclusion. 



By very careful observations under the high power it 

 was found that if the ventral surface, the surface opposite 

 the eye-spot, faces the source of light, after the direction 

 of the rays is changed, there is no immediate reaction. 

 The Euglenae continue on their course as though no change 

 had taken place until the rotation on the long axis carries 

 the dorsal surface over into a position in which it faces the 

 light. As soon as this surface, the surface containing the 

 eye-spot, faces the light there is a definite reaction. The 

 Euglenae turn the anterior end toward the ventral surface 

 more or less sharply, i.e., away from the source of light, but 

 they continue to rotate so that the ventral surface soon 

 faces the light again; but it is evident, owing to the curva- 

 ture in the body, that the anterior end is now directed more 

 nearly toward its source than it was when this surface faced 

 the light during the preceding rotation. While in this 

 position, the body is somewhat straightened, so that the 

 anterior end is not carried back as far during the following 

 rotation, and when the dorsal surface comes to face the 

 light it is directed more nearly toward its source than it 



