350 LESLIE B. AREY 



2. Ameiurus. The pigment of this fish is at first more sensi- 

 tive than that of the other animals studied, migration being 

 distinctly initiated in many (or perhaps most) individuals at an 

 intensity by which the presence of objects can be detected. At 

 the next grade (colors distinguished), expansion is well ad- 

 vanced, although the pigment does not extend the maximal dis- 

 tance, for it is dense at the cell bases, but sparse distad. In 

 light in which one can just read, the expansion is nearly com- 

 plete, but does not become maximal until the illumination is 

 sufficient to allow easy reading. 



3. Abramis. The effect of light is not apparent until it is of 

 such a strength that colors can be recognized. At this intensity 

 the pigment in about half the individuals was essentially in a 

 condition of greatest contraction; the remainder showed the pig- 

 ment well started, but not extended at most more than half the 

 way to the external limiting membrane. Owing to a sudden 

 failure in my supply of animals at the end of the season, I secured 

 but few observations on the effect of light which makes reading 

 possible; the pigment in those animals studied, however, was in 

 a state of partial expansion only, so that it appears safe to con- 

 clude that stronger illumination is necessary to allow expansion 

 to proceed to completion. 



B. Visual cells 



1. Cones. The cones of Abramis and the frog remain fully 

 elongated, in the characteristic dark positions, until the illumina- 

 tion is so increased that colors are recognized. At this intensity 

 the cones of Abramis show distinct indications of incipient re- 

 traction, while those of the frog are less than half their former 

 length. When illuminated sufficiently to allow reading, the frog's 

 cones shorten maximally; the cones of the few Abramis studied^ 

 were likewise greatly shortened, but not completely so. 



^ As on the tests on the retinal pigment of this animal, further experimenta- 

 tion weuld have been desirable, but the supply of available material suddenly 

 ceased. 



