374 



LESLIE B. AREY 



Measurements showing the relative distribution of pigment in the excised eyes of 

 Planorbis in darkness and in light. The values are mean values expressed in 

 micra and indicate: {!) the thickness of the main pigment mass {zonal measure- 

 ment); (;S) the zonal measurement plus the mean length of the pigmented pro- 

 cesses {process measurement); {3) the length of the pigmented processes obtained 

 by subtracting (/) from {2). The percentage change in the mean zonal measure- 

 ment in darkness as compared with that in light is also computed 



waste exerts an inhibitory or anaesthetic influence upon the 

 normal activity of the protoplasm of the pigment cells. Since 

 in the case of the retina it is even probable that light favors 

 catabolism, it follows that the movement of the retinal pigment 

 in the light only, must be due to the greater efficiency of light 

 as a stimulating agent. The strong stimulatory effect of light, 

 therefore, is able to break through the protoplasmic inhibition 

 caused by accumulated wastes, whereas darkness is ineffective 

 in this respect. If this reasoning is tenable, one may infer 

 that movements of the pigment would also be realized in dark- 

 ness, provided artificial circulation could be maintained. That 

 light actually is a more efficient stimulus than darkness, is 

 further suggested from the determinations of adaption times 

 {vide infra). 



(b) Determination of adaption times 



It is evident from the foregoing observations that photo- 

 mechanical changes do occur in the pigment of the Planorbis 

 retina, and furthermore, one is led to suspect that the indecisive 

 results obtained by Smith ('06) are assignable to the short 

 duration of his experiments, which, in his own words, lasted 

 but "an hour or more." Hence the pertinent inquiry as to 



