176 yournal of Comparative Neurology and Psychology. 



is 400 cm. In accordance with Weber's law, the proportion 

 between the difference and intensity on opposite sides and the 

 intensity on either side should be the same in all degrees of illumi- 

 nation. The reactions of Volvox, as recorded in these tables, 

 therefore, are not in perfect accord with this law. But in Table 

 V the threshold is smallest in the highest light intensity, while in 

 Table VI it is smallest in the lowest intensity. There was also 

 a surprising difference in the threshold of the organisms used on 

 the two different days, confirming the statement made elsewhere, 

 that the reactions of these organisms at any given time depend 

 largely upon previous environmental conditions. Considering 

 these facts, it seems almost certain that the difference between 

 the results recorded in the tables and those demanded by Weber's 

 law are within the limits of error. If this be true the light reactions 

 of Volvox may be considered to be in accord with this law. 



19. SUMMARY. 



1. The eye-spots in Volvox are located on the outer posterior 

 surface of the individuals of which the colonies are composed, not 

 on the outer anterior surface as represented by Overton. 



2. They are much larger in the individuals at the anterior end 

 than in those at the posterior end, and they probably function 

 as light recipient organs. 



3. In moving forward Volvox usually rotates on its longitudinal 

 axis counter-clockwise, as seen from the posterior end. But under 

 certain conditions the direction of rotation is frequently reversed. 

 This is caused by continuous contact stimulation of the individuals 

 located along the sides of the colonies. 



4. In swimming horizontally Volvox colonies seldom move 

 parallel with the light rays when exposed to light from a single 

 source. They deflect upward or downward as well as to the right 

 or left. 



5. Specimens containing large daughter-colonies or spores 

 deflect more strongly to the right than others. The degree of 

 deflection depends upon the light intensity and the physiological 

 condition of the organism as well as upon its contents. The more 

 strongly positive they are, the more nearly parallel with the rays 

 they appear to move as seen from above. When exposed to light 

 of very low or very high intensity they deflect more than when 

 exposed to light of moderate intensity. 



