250 HENRY LAURENS 



the central nervous system had been completely destroyed by 

 boring it out. In all, the results were the same, expansion of 

 the melanophores illuminated by the beam of light, and no ef- 

 fect on the pigment cells of the remainder of the body. 



These results show conclusively that the expansion of the melan- 

 ophores of the larvae of A. punctatum caused by light are due 

 primarily to the direct stimulation of the pigment cells them- 

 selves and not to the inhibitory action of the nervous system. 

 The nervous system certainly helps to bring about the expan- 

 sion, just as it helps to bring about the contraction of the me- 

 lanophores when the larvae are placed in darkness, for, when 

 the reaction times of normal and eyeless larvae are compared 

 it is seen that those of the former are shorter (Laurens, p. 585, 

 table 2) . The experiments on larvae in which the central nerv- 

 ous system was cut through, and partially or totally destroyed 

 (pp. 614-616) could be considered sufficient proof against the 

 assumption of the action of an inhibiting center in Amblystoma, 

 such as Fuchs suggests. But there seemed sufficient reason 

 for carrying out experiments to test just this point. These 

 have afforded us additional evidence concerning several things 

 mentioned in my former paper and have added information not 

 there contained. One interesting fact which was brought out 

 and which seems worthy of remark is that although the roof 

 of the diencephalon is cut out with the epiphysis, nevertheless 

 the secondary reactions of the melanophores, and the reactions 

 to background, which are certainly dependent on the presence 

 of the eyes, still come about. As we know, the median por- 

 tion of the diencephalon has no nervous elements in it, these 

 being in the lateral walls. In a few experiments which were 

 carried out on larvae after cutting the lateral walls, the second- 

 ary reactions of the pigment cells, as well as the reactions to 

 black and white backgrounds did not take place. 



DISCUSSION AND CONCLUSIONS 



Fuchs' idea that the parietal organ, or, as we may now say, 

 the epiphysis, and the surrounding region of the roof of the brain 

 has an inhibitory influence on the melanophores does not hold 



