256 HENRY LAURENS 



ness the pigment expands again, not because of a specific stimu- 

 lating effect of darkness, but simply because the heat stimulus 

 ceases to act. Why, after a day, these isolated melanophores 

 show exactly the reverse reactions to a heat stimulus that they 

 did before is not entirely clear to me. That it may be due to 

 an increase in the acidity of the cells, both of the pigment and 

 surrounding tissue cells, in other words to a change in the H 

 ion content, is not absolutely impossible. 



Steinach's results, judging from the results of Biedermann's 

 work which show the dependence of the chromatophores upon 

 the central nervous system, were probably also obtained by 

 illuminating the animals with sunlight. The effects therefore 

 are to be referred to the action of heat and not to light. 



The chief function of the chromatophores is probably to enable 

 the animals possessing them to adapt themselves to the intensity 

 and color of their background. To make this possible the pig- 

 ment cells must be under the control of the eyes which regulate 

 through the nervous system, the movements of the pigment 

 suspended in their cytoplasm. It is very possible that in some 

 animals the eyes may have no (or very little) influence over the 

 chromatophores as Biedermann has pointed out to be the case 

 in the frog where according to him, light plays a small part 

 in the color changes. In some animals conditions are of course 

 different from those in others. Von Frisch ('11 and '12) has 

 shown this to be the case in fishes. The minnow Phoxinus does 

 not possess in any marked degree the power of adaptation to 

 background, but the melanophores respond more to the relative 

 intensity of the light. In Salmo the response of the melano- 

 phores to different backgrounds is very marked. In Crenila- 

 brus pavo this is also the case. But the control of the nervous 

 system is in each of a different nature. In Phoxinus when the 

 eyes are removed the melanophores expand in the light and 

 contract in the dark, which is opposite to the conditions seen in 

 the normal fish. In Salmo, when blinded, the melanophores are 

 under all conditions expanded, and in Crenilabrus the melano- 



