370 LESLIE B. AREY 



then made between the two kinds of preparations in order to learn 

 whether the position of the pigment was different in the two cases. 

 More cases in the 'hght' eyes had the pigment reaching quite to the 

 nuclei than in the 'dark' eyes; but there was such a lack of uniformity 

 in different animals, and even in the same retina, that the evidence 

 was not at all conclusive. Repeated experiments did not lead to more 

 definite results. I am satisfied that the pigment does travel up and 

 down in the pigment cells under the influence of some stimulus, but 

 just what is the exciting factor I have not determined. I have not 

 been able to get any evidence of pigment migration in the eyes of either 

 Helix or Limax. 



In another part of the same paper Smith again returns to a 

 consideration of these results and becomes more bold in his 

 inferences. Selected excerpts (pp. 268-269) make plain his 

 final attitude toward the situation in Planorbis. 



It is probable that pigment movement is a direct response to light- 

 stimulation in Planorbis .... Not having been able as yet 

 to determine the exact conditions under which it occurs, I can only 

 suppose from analogy that the migration is a response to light. The 

 shape of the cells, the position of the pigment in some cells as com- 

 pared with that in others, and the apparent need of pigment migration 

 in the" eye of Planorbis, all point to a probable responsiveness of its 

 pigment cells to light; .... 



A personal experience (Arey, 16^) in determining the lengths 

 of time needed to complete light and dark adaption of the highly 

 mobile retinal pigment of fishes led me to suspect that the 

 'hour or more' which Smith allowed for the execution of his 

 experiments might be wholly insufficient to permit the photo- 

 mechanical response to proceed to completion. Hence in 

 my experimentation ample time allowances were made for both 

 light and dark adaption. 



1. Effect on normal animals. Five animals were placed in a 

 battery jar containing water and were kept in a dark-chamber 

 for 24 hours. In the same manner, an equal number of animals 

 were subjected to daylight for 10 hours. A microscopical 

 examination of the resulting preparations gave data as in table 1.^ 



1 The final measurements recorded for each eye represent the mean of the 

 values obtained from several measurements on each side of the retina at a dis- 

 tance of about 50 ij. from the entrance of the optic nerve. Experience showed that 



