INTEGUMENT OF NECTURUS MACULOSUS 521 
(three to five days) exhibited what Laurens terms ‘secondary’ 
color reactions, which are directly opposite to those already 
outlined. 
All the results that have been briefly reviewed show that 
bright illumination causes a contraction of the melanophores, 
although this effect may be offset by the color of the environ- 
ment. However, Hooker (’14a), in his experiments on the 
tadpoles of Rana pipiens, found that the dermal melanophores 
responded oppositely to those of the adult frog, expanding in 
the light and contracting in the dark. The color of the back- 
ground had no effect on the response, and continued exposure 
to darkness produced a ‘secondary’ reaction of expansion of the 
melanophores. 
2. In Necturus. Necturus shows responses to light similar to 
those noted by Hooker (14 a) in the frog tadpole. In bright 
light on a white, black, or indifferent background the melano- 
phores are always expanded. Animals placed in cool water in 
a photographic dark-room gradually become lighter in color, and 
a maximum response is obtained in about three hours. On 
their return to the light they regain their normal dark appear- 
ance in practically the same time. In bright light the color of 
the background has no effect on the nature of the response. 
But in Amblystoma, as has already been noted, a black back- 
ground causes a complete expansion of the melanophores, which 
normally, on an indifferent or white background, would be con- 
tracted. The chromatophores of Necturus in dim illumination, 
as in the late afternoon, are usually partially contracted, and 
under these conditions animals on a white background are lighter 
than those in a black or gray environment. Hence we may 
conclude that strong light is a more effective stimulus than the 
color of the background, and when both are acting at the same 
time the environment is unable to produce any noticeable effect. 
Only when the light is greatly reduced, as in twilight, do we 
find the white background able to modify the response. No 
‘secondary’ responses were obtained, although in many cases 
animals were left in the dark-room from ten to fourteen days. 
