PHYSIOLOGY OP CHROMATOPHORES OF PISHES 529 



and a diffusion (expansion) of the xanthophores. Over a dull 

 black bottom the fishes become correspondingly black, the mel- 

 anophores being fully expanded while the xanthophores tend to 

 contract. Thus at the extremes of the color phases the two 

 types of chromatophores may behave in a reciprocal manner — 

 the one expanding when the other contracts. 



The relation to the sympathetic nervous system, which has 

 been so frequently described in a variety of fishes, also occurs in 

 this species. Thus, for example, it has been possible (appar- 

 ently) to sever one of the two branches of the sympathetic sys- 

 tem in the haemal arch posterior to the body cavity, causing a 

 loss of control of the chromatophores posterior to the point of 

 the incision and on that side only (cf. v. Frisch '11 a). Com- 

 plete loss of adaptive changes follows the excision of both eyes.^ 



If a fish which is adapted to a light bottom be decapitated the 

 body becomes intensely 4ark in the course of a few minutes — 

 the time varying with different individuals. The melanophores 

 remain in this expanded condition for some hours depending, in 

 part, upon the temperature of the water, but eventually they 

 contract and the fish becomes light. Similarly the final condi- 

 tion of fishes which have died in the aquaria is the light one. 



The rapidity of the change from the extreme light to the 

 extreme dark phase in a freshly decapitated fish suggested the 

 possibility of observing the microscopic changes within the indi- 

 vidual chromatophores. Such fish were brought upon the stage 



1 It seemed of interest to determine at what age the fish begins to show the adap- 

 tive color-changes. Many observations of twelve-day embryos failed to show 

 any such change in the melanophores so long as the egg-membrane remained in- 

 tact, though they were watched both over white and black bottoms. Light was 

 thrown upon the question by the following experiment. 



July 18, 1912. From a batch of eggs of F. heteroclitus (stripped and fertilized 

 2 P.M. July 6, 1912) a number of embryos became free between 2 and 4 p.m. All 

 these young were dark dorsally when hatched, especially in the region of the brain. 

 No distinct limits of individual melanophores were visible when first they slipped 

 from the membranes. Two groups of four each were placed in white and black- 

 bottomed Syracuse glasses respectively at 4.30 p.m. At 7.30 p.m. all four fish over 

 the dark bottom were dark as when hatched while all those over the light bottom 

 were light and showed the melanophores, particularly those of the head, to be 

 contracted. Thus within six hours after hatching F. heteroclihis is capable of 

 adaptation to the ground-color of its environment. 



