1 72 journal of Comparative Neurology and Psychology. 



Fig. 15. Representation of appa- 

 ratus and arrangement as used in ascer- 

 taining the minimum difference in light 

 intensity on opposite sides which induces 

 reaction in Volvox, in various intensities 

 of illumination, a, glass aquarium 4 cm. 

 long and 3 cm. wide; b, glass tube 

 through which the colonies were intro- 

 duced; c, metric gauge; J, Nernst glower, 

 horizontal; w m, mirrors; r, light rays; 

 s, dead black opaque screens; w, water 

 screen. 



