MOTOR RESPONSES 279 



the observations on it should be repeated and extended under carefully 

 controlled conditions. 



Mast and Stabler (1937) made a thorough study of the relation be- 

 tween luminous intensity and rate of locomotion in A. pro feus. They 

 found that if dark-adapted amoebae are exposed to light, the rate of 

 locomotion gradually increases to a maximum and then remains con- 

 stant; that the time required to reach the maximum decreases from 15 

 minutes at 225 meter-candles to a minimum of 7 minutes at 15,000 

 meter-candles, and then increases to 30 minutes at 40,000 meter-candles; 

 and that the rate of locomotion at the maximum increases from 128.8 zh 

 10.8 micra per minute at 50 meter-candles to 219-3 ± 11-4 micra per 

 minute at 15,000 meter-candles, and then decreases to 150.2 ±: 8.5 

 micra per minute at 40,000 meter-candles (Fig. 96). They present evi- 

 dence which indicates that the increase in rate of locomotion with in- 

 crease in light intensity is due to the action of the longer waves, and that 

 the decrease in rate in intensities beyond the optimum is due to the action 

 of the shorter waves. This action of light on rate of locomotion is similar 

 to the action of temperature. It is probably due to changes in the rate of 

 sol-gel and gel-sol transformations. If this is true, both of these trans- 

 formations must be augmented by the longer waves and retarded by the 

 shorter. 



Orientation. — Davenport (1897) found that A. proteus orients fairly 

 precisely in a beam of direct sunlight and that it is photonegative, but 

 he did not ascertain the processes involved in orientation. 



Mast (1910) demonstrated that if an amoeba is unilaterally illu- 

 minated, pseudopods develop more freely on the shaded side than on 

 the illuminated side, and that this results in gradual turning from the 

 light (Fig. 97). He concludes that orientation is due to retardation in 

 the formation of pseudopods on the more highly illuminated side, owing 

 to increase in the thickness of the plasmagel on this side caused by the 

 gelating effect of light. 



There is some evidence which indicates that A. proteus is photopositive 

 in very weak light (Schaeffer, 1917; Mast, 1931a). More carefully con- 

 trolled observations concerning this are highly desirable. 



It is possible that the kinetic responses in Amoeba are due to changes 

 in the rate of sol-gel transformations at the anterior end, and gel-sol 



