QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 259 



produced with the blackened side towai'd the center of the circle. 

 I tried a similar experiment with larvae that showed a markedly 

 as3Tiimetrical response to equal horizontal beams of light by sub- 

 jecting them to light from above. The tendency to turn toward 

 the less sensitive side was very marked. The turning, under non- 

 directive light, of larvae that have been made artificially asym- 

 metrical in their sensitiveness to light, and the similar turning of 

 larvae naturally asymmetrical in their sensitiveness is hardly to 

 be explained on any other basis than the assumption that unequal 

 stimulation of opposite photosensitive areas is proportionately 

 transmitted to the muscles concerned in locomotion. A significant 

 fact in this connection is that the balance of reactiveness in manj^ 

 of the larvae was found to fluctuate ; a larva, which in one experi- 

 ment gave a symmetrical response, often giving soon afterwards 

 under the same experimental conditions, an asymmetrical one. 

 Figure 20 (p. 260) is a photograph of the test trails, and six pairs of 

 trails made by the same larvae under a constant intensity differ- 

 ence. The response is first 'left-handed,' b, (i.e., stronger to the left 

 than to the right, implying that the right photosensitive area is 

 more sensitive), then right handed, c, then symmetrical, d and e, 

 then right-handed again, /, and finally symmetrical, g. 



Walter ('07, p. 59) suggested that the asymmetry of response ^ 

 to light that he observed in planarians was due to internal 

 irregularities. Undoubtedly asymmetry of response depends in 

 many cases on anatomical differences in the opposite sides of 

 an animal. In such cases the asymmetry probably does not 

 change markedly in repeated reactions. But in cases like the 

 one cited, where the stronger response is first on one side and 

 then on the other, the balance which is disturbed must be a 

 physiological one. We may assume that the processes of metab- 

 olism, or the previous reactions, or m short any of the factors which 

 Jennings regards as effective in altering the 'physiological states,' 

 have produced a change in the relative abundance or instability 

 of the photosensitive chemicals of the receptive areas. Such an 

 assumption is not unreasonable in view of the well known ex- 

 periments of Loeb on the modifiability of light reactions by the 

 use of chemicals; and the inducing by similar means of a light 



