138 LESLIE B. AREY 



111 th(^ (lark the contraction is very incomplete and some- 

 times tends to give rise to a distal accmnulation of pigment 

 which faintly resembles the distribution in the light, although 

 such a condition is not constantly present as a characteristic 

 of dark adaption. It is impossible to state the cause of this 

 pigment massing. It may be due to a greater activity at the 

 distal ends of the cells in producing a contraction, or what would 

 bring about a similar end result, a contraction of pigment en 

 masse. 



Measurements were taken of preparations at the extreme 

 temperatures in the dark, the degree of variation being greater 

 than in any of the other fishes studied. Thirteen retinas at 

 5°C. (fig. 7) showed the pigment to be extended a mean dis- 

 tance of 20.4 divisions of the ocular micrometer, whereas fif- 

 teen retinas at 25°C. (fig. 8) had a corresponding value of 14.4. 



3. Abramis. In the hght the condition in the eye of this 

 fish is somewhat similar to that in Fundulus. The pigmenta- 

 tion is very heavy, forming a broad zone near the external limit- 

 ing membrane; between this distal zone and the base of the 

 pigment cells pigment granules are also present, the density of 

 pigmentation depending on the temperature. 



At 5°C. (fig. 9) the distal zone is wider than at 25°C. (fig. 10) 

 in the ratio of 50 n to 38 /z, while the proximal area is much less 

 heavily pigmented than at 25°C. Although each area is sharply 

 defined, the one appears to grow at the expense of the other. 

 It was not possible to get so complete a distal accumulation 

 of the pigment as in Fundulus and, as will be presently shown, 

 the extent of migration in Fundulus is still less complete than 

 that in Carassius. 



The dark phase is one of great contraction, and although 

 some retinas show evident differences at the extreme tempera- 

 tures, yet throughout the whole set judgment of the eye has 

 to be supplemented by actual measurements. Such measure- 

 ments show that the mean expansion at 5°C. (fig. 11) exceeds 

 that at 25°C. (fig. 12), the values being 30 /x and 20 ^ respectively. 



4. Carassius. The eye of the goldfish is more heavily pig- 

 mented than those of the three other fishes. At 5°C. in the 



