426 THE PREFUNCTIONAL AS CONTRASTED WITH 



also causes extra multiplication of melanophores.^ A similar 

 result, doubtless brought about in the same way, is seen in fish 

 (Lebistes). Specimens reared on white backgrounds have con- 

 tracted melanophores, few in number; specimens reared on 

 dark background have expanded melanophores in large numbers. 

 Functional activity increases the rate of multiplication^ (fig. 206). 

 Similarly in salamander larvae (S. maculosa), yellow backgrounds 



$ 



y. ...... .M 



4 



Fig. 206 

 Functional activity and rate of multiplication of pigment-cells. Dorsal view of 

 the trunk region of two specimens of the teleost fish Lebistes retkulatus, one (i) 

 reared for 6 months on a white background, the other (2) for the same length 

 of time on a black background. In both cases the pigment-cells (melanophores) 

 have been induced to assume the contracted state by adrenalin treatment. Note 

 the much larger number of melanophores in the black-adapted specimen, in which 

 during life they were expanded normally, while in the white-adapted specimen 

 they were contracted. (From Sumner and V^eWs, Journ. Exp. Zool. LXiv, 1933.) 



favour the increase of the yellow areas, black backgrounds that of 

 the black areas. After metamorphosis, however, a gradual regula- 

 tion towards the control type sets in, indicating that what we may 

 call "functional multiplication" of pigment-cells is only important 

 in certain stages.^ 



A sudden change in the activity of a gland may take place at a 

 certain stage in development, as occurs with larval Urodela, in 

 which the sudden onset of metamorphosis is brought about by the 



^ Smith, 1920. 2 Sumner and Wells, 1933. 



3 Herbst, 1924. 



