568 R. A. SPAETH 



Eight scales were used, four were exposed and four were con- 

 trols. In each case two of the four were placed face up and two 

 were inverted. The exposure lasted twenty-one minutes and at 

 the end of this time not a single illuminated scale showed any 

 sign of a contraction in the melanophores. 



Finally experiments were made to test the reversibility of this 

 ultra-violet light response. 



May 7, 1912 {continued) 



Scales from the 2.15 p.m. set were exposed in a quartz-bottomed cell 

 with an air-blast as before. The exposure began at 3.40 p. m. After 

 ten minutes a very noticeable contraction could be seen in the melano- 

 phores of the inverted scales. The cell was now removed and, with- 

 out touching the scales, it stood exposed to normal daylight. The 

 melanophores of both inverted scales began expanding at once and this 

 expansion was in exactly the reverse order of the contraction by light, 

 that is, it began at the periphery and worked towards the post-median 

 region where the light had first contracted the melanophores. The 

 melanophores which had shown the first response to light were thus the 

 last to recover. Ten minutes after removal from the light about one- 

 half of the melanophores were partially expanded (fig. 2, pi. 4). One 

 scale was now fixed. In the other inverted scale the melanophores 

 showed a complete expansion in twenty minutes (fig. 4, pi. 4). 



Possible sources of error might be sought in these experiments 

 (1) in an evaporation of the NaCl solution and (2) in an increase 

 in the dissociation of the NaCl due to the ultra-violet light. 

 Since both of these conditions would actually favor an expansion 

 of the melanophores their contraction in ultra-violet light could 

 not be explained on this basis. 



From the above experiments it seems probable that the mel- 

 anophores, when immersed in NaCl solutions of different concen- 

 trations, are not sensitive to exposures to varying intensities of 

 visible light in the region of wave-length 430-490 ixix. When 

 exposed to ultra-violet light (185-290 ^l^i) in a 0.1 M NaCl solu- 

 tion, a rapid and reversible contraction of the melanophores fol- 

 lows. Thus, under these conditions, light does not act as a 

 direct stimulus upon the melanophores except in the ultra-violet 

 region of the spectrum. 



