REACTIONS OF MELANOPHORES OF AMBLYSTOMA 257 



phores expand in the light and contract in the dark just as they 

 do in the normal fish, only more extensively. 



The reactions of Amblystoma larvae to different backgrounds 

 show that this is an important function of the melanophores. 

 Amblystoma larvae are nearly always found in pools in the woods 

 in which the bottom is covered with leaves which are either 

 dark brown or black. Occasionally they may be found in ditches 

 in a field, but here the bottom is of black mud. Over such a 

 background, as my experiments have shown, the larvae are 

 very dark, almost black, due to the complete expansion of the 

 melanophores. But over a white background the larvae are 

 pale, exhibiting, to be sure, not such a marked adaptation, but 

 at any rate the melanophores are contracted. Also over an 

 indifferent background, when the illumination is constant and 

 bright, the melanophores contract (| to j expansion), so 

 that the larvae are more nearly of the color of the bottom. 

 The fact that the time required for the changes in the melano- 

 phores to come about is long and that therefore the usefulness 

 to the individual in the way of protection, of the ability of the 

 melanophores to change according to the background, is of 

 doubtful value, is an argument that can be answered by the 

 statement that probably such conditions in change of background 

 are not very likely to occur in nature. But the fact that if they 

 do occur the melanophores can react to the change shows that 

 the ability to change is in the nature of an adaptive one. I 

 have carried out experiments in a large aquarium, directly in 

 front of a window in my room, to further test this matter. In 

 one series of experiments the bottom of one half of the aquarium 

 was of black mud or of black leaves, the other half of whitish 

 sand, the larvae being free to swim from one to the other. Under 

 these conditions the larvae are of course not at all adapted to 

 the background for they do not remain in any one portion of the 

 aquarium long enough for the melanophores to react adaptively. 

 But if the aquarium is divided by a glass partition placed at the 

 dividing line between the two different colored bottoms then the 

 adaptive changes of the melanophores which have been described, 

 do come about and the coloration of the larvae is strikingly 



