258 HENRY LAURENS 



similar to the bottom, particularly in the case of the black con- 

 dition, so that they are very difficult to see. If the partition 

 is now again removed so that the larvae can swim through the 

 whole aquarium freely then the difference in coloration between 

 the larvae that have been over the one or the other background 

 is very apparent. 



The theory that the melanophores in an aquatic animal can 

 have any function in regulating the temperature has been shown 

 by Bauer ('14) to be highly improbable. Also it does not seem 

 possible to suppose that the contraction or expansion of the pig- 

 ment cells in the skin has anything to do with the reception of a 

 light stimulus by sensory nerve endings. It is well known that 

 phototactic reactions can be induced in animals, which have been 

 deprived of their eyes, by illuminating the skin. The amphib- 

 ians show this faculty very well (Parker '03, Laurens '11). 

 But as Parker ('09) has shown only a very few fish show this 

 ability on the part of the integumentary nerves stimulated by 

 light to result in phototactic responses. These reactions are 

 supposed to be started by the action of light on receptors in the 

 skin, either directly as light energy (heat) or indirectly as chemi- 

 cal energy set free by the photochemical changes started by the 

 action of the light on the skin, which stimulate the sensory 

 nerve endings. The fact that in most fishes no such reactions 

 take place, although the pigment motor function is well developed, 

 argues against the pigment cells having any such universal func- 

 tion in aiding or retarding the rapidity of perception. Experi- 

 ments were carried out by me ('14) to test just this point as to 

 whether the condition of the pigment cell — contracted or ex- 

 panded — had anything to do with the sensitiveness of eyeless 

 Amblystoma larvae to light with negative results, and the con- 

 clusion was reached that the condition of the pigment in the skin 

 melanophores had nothing to do with the sensitiveness of the 

 larvae to light, although it was dependent upon the fact as to 

 whether they had been kept in light or in darkness before their 

 phototactic reactions were tested. 



To conclude then this brief and fragmentary discussion it 

 seems to me that the reactions of the pigment cells in the skin 



