BRADLEY M. PATTEN 455 



inated the balance of the receptive mechanism as a whole is less dis- 

 turbed than when the median eye on one side is the only functional 

 receptor. 



It is not pertinent here to enter into a detailed comparison of these 

 results on the scorpion with the experiments of Garrey (6) on the 

 positively phototropic robber fly, but the complete agreement of the 

 two series of observations made by different methods, on animals 

 having opposite signs of phototropism, is too striking to pass without 

 comment. 



Further evidence that balanced reactions depend on the functional 

 symmetry of the receptive mechanism is furnished by the measure- 

 ments made on animals in which the photoreceptors had been sym- 

 metrically interfered with. As in the case of asymmetrically sensitive 

 scorpions, the symmetrically bhnded animals were brought into the 

 field of equal opposed lights "in orientation" {i.e., with their plane of 

 symmetry perpendicular to the line connecting the sources of light) . 

 No matter what ehminations were made, so long as the photoreceptive 

 mechanism was left in a functionally symmetrical condition it suf- 

 ficed to maintain^ a balanced response which was accurate within the 

 limits of variability exhibited by normal animals under the same con- 

 ditions of illumination (Table IV and Fig. 4). 



Observations already published by many different investigators 

 have covered nearly every phase of the correlation existing between 

 bilaterally balanced excitation of photoreceptors and balanced loco- 

 motor responses. A constantly increasing accumulation of experi- 

 mental evidence indicates that the attaining and maintaining of ori- 

 entation to light depends, as postulated in Loeb's muscle tension 

 theory, on the transmission to the muscles of locomotion of impulses 

 which are proportional bilaterally to the excitation of symmetrically 

 located photoreceptors. 



The results of the experiments described above indicate that the 

 muscle tension theory appHes to the complex receptive mechanism of 



■* It has already been pointed out that when brought into the field of hght out 

 of orientation (as in the experiments under anterior and lateral illumination) 

 .symmetrically blinded animals exhibited a retardation in their rate of coming 

 to a new direction of orientation. 



