532 ADAPTATIONS TO PHOTIC QUALITY 



Thus in the ocular control of dermal response to the shade of the 

 background, the upper half of the retina acts positively to contract the 

 chromatophores, and the lower half of the retina acts in a negative way 

 to prevent such contraction. A blinded fish darkens in the light because 

 there is no eye to inhibit the innate tendency of an illuminated melano- 

 phore to expand. In other words, C (above) becomes impossible. 



Frisch found, in trout, that blacking out one eye led to a darkening of 

 only one side of the fish — the opposite side, because of the total decus- 

 sation of the optic nerve fibers in the chiasma (Fig. 21, p. 47). Sumner 

 did not find this response in the species with which he worked. A fish 

 with one eye covered took on a shade intermediate (for a given back- 

 ground) between a normal fish and one with both eyes covered. Either 

 eye ordinarily can control all of the melanophores, which seems to dem- 

 onstrate an interesting phase of binocularity in piscine ocular physiology : 

 despite the total decussation of the optic nerves, each retina has connec- 

 tion within the brain with both halves of the central nervous system. The 

 unilateral response in the trout (and other fishes) seems to be the best 

 kind of evidence for nervous control of the melanophores. No hormone 

 could very well remain only on one side of a vertebrate's body. 



Various attempts to confirm and study the 'polarization' of the retina 

 by inverting the illumination, rotating the fish, or destroying either half 

 of the retina have been successful. Not so, most efforts to rotate the eye 

 of a fish 180° in its orbit without killing the animal. Butcher, however, 

 has succeeded with this operation in Fundulus, and finds that the fish 

 will then give its tawny response to a yellow background only when the 

 latter is above the animal. 



In general, dermal responses to hue exhibit no polarization at all. That 

 is, no contrast between the upper and lower parts of the visual field is 

 required. This seems particularly interesting when one recalls the con- 

 tention of some workers, that the colored chromatophores of fishes are 

 not controlled through the nervous system. Sumner got the same yellow- 

 ing of his fishes when corneal caps were applied whose upper halves were 

 yellow, black, blue, or clear — so long as their lower halves, admitting the 

 light reflected from the white substrate, were yellow. With an all-red 

 covering, the fish took on the same dermal color as when in a red con- 

 tainer with its corneas naked. 



Morphological Color Changes in Teleosts — At the present time it 

 is the morphological color changes which hold the stage of interest. 

 Occurring in the same directions of darkening and paling, under the 



