PHOTOMECHANICAL CHANGES IN THE RETINA 537 
separable in the blinded area. Examination showed the pigment 
in this region to be densely aggregated in processes which ex- 
tended to the external limiting membrane, while in the remainder 
of the retina the pigment only reached to the inner members 
of the rods. Although these experiments were pathological, 
Czerny’s statements show that he fully appreciated the potency 
of light in producing pigment migration.! 
To Boll (77) and Kithne (’77), working independently, be- 
longs the credit of the first demonstration of the normal re- 
sponse of retinal pigment when stimulated by light. They 
established this fact conclusively by direct comparisons of 
light- and dark-adapted retinas of the frog, in which the pigment 
was observed to be highly expanded in the light but contracted 
in the dark (ef. figs. land 2). Later observations have corrobo- 
rated Kithne’s original view that pigment migration is not due 
to the extension and retraction of cell processes, but to the 
movement of pigment granules in the protoplasm of relatively 
fixed cells. 
Kiihne’s statement that a phototropie action on retinal pig- 
ment would be found in all vertebrates has not yet, in the case 
of most reptiles and mammals at least, been substantiated. 
Among fishes and amphibians a response of the pigment to 
light seems general. Thus Stort (86) on Perea, Pergens (’96) 
on Leuciscus, Hess (’02) on the marine eel, Exner und Januschke 
(05) on Abramis, and Garten (’07) on the ‘Weissfisch’ have 
observed strong distal migration in the presence of light, 
while Ktthne (’77) and Boll (77) on the frog, Arcoleo (90) on 
the toad,? and Angelucci (’78) and Stort (86) on Triton found 
the existence of a similar condition. 
The results on reptilian eyes, involving the query of Boll (81) 
as to whether pigment migration occurs in rodless retinas, have 
been somewhat contradictory. Boll himself, working on the liz- 
' Deutschmann (’82) repeated Czerny’s experiment but did not find evidence 
of an extreme pigment migration. He says (p. 250)—‘‘doch schienen zwischen 
den zerstérten Elementen der Stibchenschicht mehr als gewohnlich feine Pig- 
mentkérchen zwischengelagert.’’ 
* Although Arcoleo experimented upon pithed toads only, striking responses 
were nevertheless obtained. 
