78 HENRY LAURENS AND J. W. WILLIAMS 



position of pigment is concerned. In figure 3 the pigment is 

 further forward than in figure 4, and in addition the basal layer 

 of pigment is thinner. When figure 3 (light eye) is compared 

 with figure 2 (dark eye) the condition mentioned in our prelimi- 

 nary communication is very apparent. If it were not for the 

 relative thicknesses of the basal layer of pigment it would be 

 very difficult (disregarding the cones) to say whether the one 

 or the other was a ight or a dark eye. 



In a few cases the pigment in light eyes has been observed to 

 occupy an extreme position, the pigment needles being as far 

 forward as the ellipsoids of the rods. The average of measure- 

 ments, however, of light and dark eyes give a distance of 36 n 

 from the choroidal edge of the pigment epithelial cells to the 

 farthest pigment needle in light eyes, and of 29 /x in dark eyes, 

 so that we have an average pigment migration of about 7 fx. 



In all these cases, whether the pigment showed the expected 

 light or dark position or not, the characteristic difference in the 

 length of the cones was present. Why pigment migration did 

 not take place although cone contraction did must remain un- 

 answered. It may have been due to the fact that optimal 

 conditions were not offered in the length of time of exposure, etc. 



In the transplanted eyes the pigment migration is even greater 

 in extent than in the normal eye. Figures 5 and 6 are of a 

 light and dark transplanted eye respectively. The average of all 

 measurements made of transplanted light and dark eyes, gives 

 a distance of 12 /x as the extent of the migration of pigment. 

 Why the extent of the migration should be greater in trans- 

 planted eyes than in normal is not clear. Detwiler ('16) found 

 that in the eye of the turtle there was what he called a loss of 

 tone when the optic nerve was cut. Both the pigment cells 

 and the cones seemed to relax in that the pigment extended 

 down further (partial light position) and the cones stretched 

 (partial dark position) though there was no great difference in 

 the amount of movement occasioned by light and darkness as 

 compared with that in normal eyes. Arey ('16 b) has obtained 

 some very interesting results with the retina of Ameiurus. He 

 found that when the optic nerve only is severed that the char- 



