QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 233 



which way it turns, and the only means of maintaining its course 

 is by a balance in the sensitiveness of the photo-receptive areas. 

 This is clearly shown by trails such as those of figure 5. The 

 guiding effect of the shadow is sufficient to obscure the unbalanced 

 response in the trails of the test sheet and in the part of the trail 

 made under the influence of the single orienting light. These 

 trails lie almost exactly in the direction of the rays. But when 

 the equal side lights are thrown on, which with a balanced larva 

 should produce a direct continuation of the trail made under the 

 influence of the orienting light, the asymmetry becomes at once 

 evident in the deflection to the left. This larva, which gave a 

 symmetrical test, proved to have the right side more sensitive 

 than the left. Owing to some bilaterally unbalanced factor within 

 the organism itself, it behaved, under the influence of equal 

 lights, as if the right-hand light were stronger. Hence the 

 deflection of a single trail is of little value in estimating the light 

 effect. But turn the larva around and make it crawl into the 

 field of light from the opposite direction (fig. 5, b), it still swings 

 toward its own left side, but by reversing the direction of the 

 crawling, the expression of asymmetry has been made to fall on 

 opposite sides of the perpendicular to the line connecting the 

 sources of light. If we measure deflection to the right in plus 

 degrees and deflection to the left in minus degrees and add the 

 two, the trails of an asymmetrical larva, run in pairs in opposite 

 directions, are reduced to the theoretical response, and to the 

 actual response given by perfectly symmetrical larvae. 



7. Measurement and tabulation 



The trails are measured as shown in the diagram (fig. 6) . The 

 lines xy and x'y' are drawn through the trails at the point where 

 the larva was when the side lights were turned on, and perpendicu- 

 lar to the line connecting the sources of light. A protractor is 

 laid on the trail, with its center at the point where the lateral 

 lights were turned on, and the deflection is measured in degrees. 

 I have called this the angular deflection of the trail. 



The measurements obtained from each individual were col- 

 lected in tables such as table 2. The deflection of each trail is 



