254 HENRY LAURENS 



of the nervous system, which they normally are, it may be, ow- 

 ing to the nature of things, environmental conditions (back- 

 ground) or psychic factors, etc., that the sense of the reaction 

 to a light stimulus is reversed and contraction is brought about. 



What causes the melanophores to contract in darkness is 

 another matter, it happens whatever its cause may be. From 

 the conflicting evidence that we have we are forced to specu- 

 late regarding its cause. It may be due to chemical substances, 

 such as Fuchs suggests, which arise as a result of the ordinary 

 processes of life (inner secretions, or what not) . It may be due 

 to chemical changes in the skin, brought about by darkness, 

 (or the absence of light). It may be due, on the other hand, in 

 some cases at least, to what may be called for want of a better 

 term, a simple relaxation, for the reason that when pieces of the 

 skin are removed from some animals (the frog, Biedermann; 

 Anolis, ^Carlton) the melanophores contract. (Laurens, '15, 

 p. 599). But that this contraction may also be due to chemi- 

 cal changes is perfectly possible. 



The expanded condition of the melanophores in darkness which 

 comes about in animals in which, due to the control of the nerv- 

 ous system, the pigment cells contract in light, is probably not 

 a specific reaction to darkness but one which is due to the lack 

 of the nervous impulse which light sets up. The pigment cells 

 have become subordinated to the influence of the nervous sys- 

 tem, and when the condition of excitation ceases (in darkness) 

 the melanophores expand. It is interesting, in this connection, 

 that Hargitt could observe, in tree frogs, no definite effect of 

 darkness (and of low temperature) on the pigment cells. 



The contracted condition of the pigment cell is claimed by 

 many to be its active state. Arguments are put forward to sup- 

 port this view from the fact that muscle contracting stimuli 

 also cause the chromatophores to contract. But light (and 

 under certain conditions high temperature) is the adequate stim- 

 ulus for the chromatophores of animals and light causes the 

 melanophores in many animals to expand, both when under the 

 control of the nervous system and when released from this 

 control, perhaps in all when the influence of the nervous sys- 



