424 FRANK W. BANCROFT 



axis. But in all of them a certain amount of forward swimming 

 was possible and in all of them negative heliotropism occurred in 

 spite of the fact that in the drops containing seven-eighths of the 

 jelly the speed of revolution about the long axis was reduced from 

 once in half a second to once in 2| or 3 seconds. When the jelly 

 was made so stiff that no swimming was possible the changes in 

 the shape of the body were so extensive that nothing about the 

 effect of the light on the flagellum could be made out. 



C Conclusions 



So far as I know the only evidence that Jennings and Mast have 

 presented to show that the stimulus which is responsible for the 

 gradual heliotropic orientation of Euglena is a change in light 

 intensity is: 



1. The stimulus for the motor reactions is a change in the light 

 intensity. 



2. The gradual heliotropic orientation is essentially a series of 

 motor reactions. 



3. The sudden swerving which produces the gradual heliotropic 

 orientation is accompanied by changes in the light intensity, and 

 therefore may conceivably be produced by these changes. 



The main evidence and the main argument is included under 

 2. Consequently with the demonstration in the previous chapter 

 that the gradual heliotropic orientation depends upon quite a 

 different mechanism from the motor reactions all of the real evi- 

 dence presented by these authors falls to the ground. 



On the other hand it has just been shown that in favor of the 

 view that the gradual heliotropic orientation is a function of 

 the continuous action of the light we have the direct evidence 

 that: 



1. The continuous action of the light is capable of producing 

 a continuous widening of the spiral which lasts as long as the light. 



2. The continuous action of the light keeps the Euglenae 

 oriented in the face of influences which tend to destroy the orien- 

 tation. 



