262 BRADLEY M. PATTEN 



the side artificially made less sensitive. (5) Asjrminetry of sen- 

 sitiveness may be balanced by a corresponding inequality of the 

 stimuli acting on opposite sides of the animal. 



There seems to be no explanation tor the response of the 

 blowfly larva to opposed lights other than the assumption that 

 symmetrical sensitive areas operate on the musculature of the 

 two sides of the animal in proportion to the stimulation received. 

 We do not know precisely what mechanism is concerned in the re- 

 action, nor even what it is that is 'balanced' in the receptors. 

 There are, however, certain general lines on which such a mechan- 

 ism must be based. If the angle of orientation under opposed 

 beams of light is such that the stimulation of the opposite sides is 

 equalized, the receptive mechanism must be of such a nature that 

 varying the axial position of the animal produces changes in the 

 relative amount of stimulation received on opposite sensitive 

 areas. Otherwise there would be no cause for the animal to as- 

 sume a definite angle of orientation for each intensity differ- 

 ence between opposed beams of light. This equalization cannot 

 be accomplished by a median sensitive area unless we assume that 

 the area operates differentially on opposite sides of the median line, 

 an assumption which throws us back again to bilaterality. Nor 

 can it be accomphshed by bilaterally located sensitive areas that aie 

 parallel to each other, nor by fixed eyes so placed that the 

 tangents to the eyes at the optical axes are parallel. This can be 

 made clear by a diagram such as figure 21. The heavy black lines 

 represent light-sensitive areas of, let us say, 1 sq. cm. of surface. 

 In figure 21, a, the sensitive surfaces, being perpendicular to the 

 rays of light, intercept 1 sq. cm. of the light as represented by their 

 projection on a plane at right angles to the rays of light (double 

 hues of fig. 21, a). In figure 21, h, the planes have been rotated 

 through an angle of about 45 degrees. This cuts down the amount 

 of light falling on each sensitive surface. But since the surfaces 

 are parallel, their projection on a plane at right angles to the 

 rays of light will be the same, and the amount of light falling on 

 the two sensitive surfaces will still, be equal. The case illustrated 

 by the diagram, figure 21, c, is essentially like that of figure 21, b. 

 The retinas will receive light proportional to that which falls 



