220 LESLIE B. AREY 



optic nerve was severed, was shown to be non-existent. It was 

 also sliown that roughly performed operations, in which various 

 mutilations of the eye muscles and the adjacent blood vessels 

 were intentionally made, did not interfere with the obtainment 

 of essentially typical results. 



This type of experiment has been repeated very many times. 

 Among animals used during both the spring and the fall of 1913, 

 no deviation was found from the conditions recorded above. 

 When work was begun again on a new supply of Ameiurus 

 in the spring of 1914, the results secured from operated animals 

 brought from darkness into the light, were not always as decisive 

 as those of the preceding year. Some experiments showed a 

 complete retention of the pigment, while others resulted in a 

 partial migration, which, in some cases, was quite extensive 

 although never as complete as in normal light adaption. Further 

 experimentation on other animals procured in the fall of 1914 

 also gave inconsistent results, varying from complete to very 

 incomplete control. 



I am unable to state the cause of these discrepancies. That 

 the results of the work done in the year 1913, which embraced 

 the larger part of this experimentation, are beyond question, I 

 feel confident; the remarkable consistency of a long series of 

 experiments about to be described substantiates this conviction. 

 On the other hand, the persistent occurrence of more or less 

 incomplete, mingled with complete control in the work of the 

 year 1914, was also undeniable. 



It is possible that individual fish vary considerably in their, 

 ability to inhibit the migration of pigment after sectioning of 

 the optic nerve, or what amounts to the same thing, the activity 

 of the pigment in producing expansion may be variable. I shall 

 attempt to present evidence, based on experimentation to be 

 described later, that demonstrates an inhibiting mechanism which 

 prevents the pigment from migrating when the optic nerve is 

 severed. Since it has been proven (Arey, '16) that the action 

 of light on completely excised eyes can directly stimulate the 

 retinal pigment to movement, it is evident that there must be 

 a competition between light stimulation and this inhibiting mech- 



