36 COLOR CHANGES IN ANIMALS 



pigment in certain melanophores at the edge of its band 

 will be found to be as fully dispersed as those on the tail 

 in general. When this fish is made to blanch by a short 

 retention in an illuminated white-walled vessel these 

 same melanophores will be found not to have concen- 

 trated their pigment fully but to have come to rest with 

 their coloring matter in a position between the extremes 

 of concentration and of dispersion; in other words, these 

 melanophores can disperse their pigment fully but can- 

 not concentrate it fully. In a corresponding way other 

 melanophores can be found also at the edge of the band 

 that can concentrate their pigment fully but cannot 

 disperse it fully. These conditions, so far as the band 

 as a whole is concerned, may cause its edge to appear 

 to shift slightly, depending upon whether the tail as a 

 whole is dark or pale. An understanding of this pecul- 

 iar situation is possible on the assumption of double 

 innervation, but not on that of single innervation. 

 When nerve-strands are cut, as in the excitation of a 

 caudal band, both kinds of fibers, assuming both kinds 

 to be present, must of course be severed. Nerve-fibers 

 in the tail do not pass out into that structure on strictly 

 radial lines, but scatter somewhat irregularly. Conse- 

 quently near the edge of a caudal band it would not be 

 surprising to find certain melanophores whose concen- 

 trating fibers had been eliminated by the cut, but whose 

 dispersing fibers were still intact, and others in which 

 the reverse was true. Under these circumstances some 

 melanophores would be open to excitation for dispersion 

 but not for concentration and vice versa, a condition of 

 affairs that would result in exactly what has been ob- 

 served. Double innervation then will explain this pe- 

 culiar state; single innervation will not. 



Another aspect of the problem of double innervation 

 is found in the regeneration of chromatophoral nerves. 



