NEUROHUMORS 



47 



hand was, but about halfway out on the rays toward 

 the tip of the tail, the result will be a central pale band 

 whose distal half will be abutted laterally by dark bands. 

 The proximal half of the pale band will be surrounded 

 only by the pale portions of the tail. Under such con- 

 ditions the distal half of the pale central band which is 

 flanked by the dark half-bands 

 will be seen to darken slowly, 

 whereas the proximal half will 

 remain light (Fig. 34). This 

 experiment shows quite clearly 

 how adjacent dark areas may 

 bring about a dispersion of 

 pigment in a pale area, a 

 change easily understood from 

 the standpoint of an invading 

 dispersing neurohumor. The 

 darkening just described occurs 

 only when the invaded region 

 of the pale band is a denervated 

 one. If the dark half-bands are 

 excited on either side of an 

 innervated pale ray, no such 

 deepening of tint occurs (Fig. 

 35). Apparently the normal 

 concentrating fibers of such an 

 innervated area are too active 

 in maintaining the pale state 

 to be overcome by an invading dispersing neurohumor 

 (Parker, 1934^). 



According to this general view, then, the two sets of 

 melanophore nerves in Fundulus act on its color-cells 

 through appropriate neurohumors which are produced 

 by the proper nerve terminals and which excite in one 



Fig. 34. Diagram of 

 the caudal tin of a hy- 

 pophysectomized catfish 

 with a faded caudal band 

 between two newlv cut 

 dark half-bands. The 

 half of the faded band 

 not flanked bv the new 

 dark bands has remained 

 pale; the half flanked by 

 the dark bands has dark- 

 ened slightly. Parker, 

 Jour. Exp. Zoo/., 1934, 

 69, pi. 1, fig. 5. 



