MOVEMENTS IN THE VISUAL CELLS 139 



light (fig. 13) all the pigment is located distalh" forming a broad, 

 dense zone. Between this band and the base of the cells is a 

 narrow clear area in which scattered pigment granules are visible 

 only with the aid of high magnifications — for practical purposes 

 it may be said to be free of pigment. 



The situation at 2o°C. in the Hght (fig. 14) is variable. In some 

 cases it closely simulates that at 5°C'., although the clearer 

 space is always relatively more heavily pigmented; in other 

 instances, the pigment is uniformly distributed from the proxi- 

 mal to the distal extent of the cells. 



The contraction that usually occurs in the dark was less pro- 

 nounced in C'arassius than in any fish heretofore described; in 

 fact in some eyes that had been subjected to a low temperature, 

 the actual l)readth of the pigment layer nearly equalknl that of 

 a light-adapted eye. The relative expansion at the extreme 

 temperatures in the dark, however, leaves no doubt that at 

 5°C. (fig. 15) the contraction is less than at 25°C. (fig. 16). 

 Measurements (in terms of the divisions of an ocular micrometer) 

 taken from eight eyes at 25°('. and ten eyes at 5°C\ gave re- 

 spective mean values of S.O and 12.1. 



From observations on these four genera certain generaliza- 

 tions are suggested. The degree of pigmentation in the eye of 

 Ameiurus (figs. 1, 2) is much less extensive than in the other 

 fishes, as a comparison with Carassius (figs. 13, 14) shows in 

 a striking manner. The three other fishes, however, offer better 

 opportunities for comparison since their pigmentation is more 

 nearly equal. At o°C. in the light the pigment of both Carassius 

 and Fundulus (figs. 13, 5) migrates distally to such an extent 

 that a proximal zone, devoid of pigment, is created. Moreover, 

 the dark phase of the pigment, in both these animals (figs. 16, 8), 

 is one b}^ no means extreme when compared with the highly 

 compact layer in .Vbramis and Ameiurus (figs. 12, 4). In the 

 last named fishes, on the contrary, it is impossible under the most 

 favorable conditions of light and temperature to obtain the 

 proximal clearer area entirely free from pigment; correlated 

 with this absence of complete expansion is the high degree of 

 contraction which is evoked in the dark. 



