AHUHKVIATIONS 



cll. bac, roil cUipsuiil ///-.s. dtit. bac, outer incmljcr of rod 



ell. con., cone ellipsoid /)/-.s-. dst. con., outer member of cone 

 7nb. Urn. e.i-., extornal liini1iii<i mctii- nix. n. opt., root of optic nerve 



Itraiio rln., retina 



t)i!/. bar., rod inyoid scl., sclera 



mi/, con., cone myoid st. bac. con., layer of rods and cones 



n. opt., optic nerve st. nl. ex., external nuclear layer 



pd. cl. pig., base of pi^nicnt cell st. pig., pigment layer 



PLATE I 



EXPLANATION OF FIGURES 



The figures of this plate are photomicrographs. Figures 1, 2, 3, and 4 are 

 magnified 160 diameters, figure 5, 46 diameters, figure 6, 19 diameters, and fig- 

 ure 7, 50 diameters. 



1 Shows the distribution of the retinal pigment of Ameiurus at 5° C. in the 

 light. 



2 Shows the distribution of the retinal pigment of Ameiurus at 25° C. in the 

 light. 



3 Shows the distribution of the retinal pigment of Ameiurus at 5° C. in the 

 dark. 



4 Shows the distribution of the retinal pigment of Ameiurus at 25° C. in the 

 dark. 



5 A portion of a section passing through the retina of a previously dark- 

 adapted Ameiurus. In the region marked X, close to the optic nerve, a smal cut 

 had been made through the eyeball and retina. When the fish was subjected to 

 daylight for 1| hours, the pigment peripheral to the incision did not expand, as 

 is shown at the right of X in the figure. The pigment in the regions of the retina 

 the optic nerve fibers of which were not affected by the incision expanded essen- 

 tially in a normal manner, as is shown in the left half of the figure. 



6 A section through the entire retina of Ameiurus. After the optic nerve of 

 the previously dark-adapted animal had been one-half severed, the animal was 

 exposed to daylight for 2 hours. In the half of the retina at the right of the 

 figure, which was adjacent to the intact portion of the nerve, the pigment 

 migrated to an extreme distal position, leaving an area behind relatively free 

 from pigment. In the left half of the retina, which was adjacent to the cut 

 side of the nerve, the pigment migrated, but not as completely as on the other 

 side. This section passed close to the optic nerve but not through it. 



7 A portion of a radial section through the retina of Ameiurus. After the 

 optic nerve of a previously light-adapted animal liad been two-thirds severed, the 

 fish was subjected to total darkness for 2 hours. At the expiration of this time, 

 the pigment in the half of the retina adjacent to the cut was found to have re- 

 mained in the expanded position characteristic of light (shown at the right of 

 the optic nerve in the figure), whereas in the half of the retina adjacent to the 

 intact side of the nerve, normal pigment contraction occurred (shown at the 

 left of the optic nerve in the figure). 



