88 KINETIC HORMONES — I 



to an illuminated black background, on which the chromatophores 

 will become fully dispersed (index 4.5), and another group to an 

 illuminated white background, on which they will become fully 

 concentrated (index 1). If the backgrounds are then reversed and 

 observations are made on the state of the chromatophores on a 

 chosen part of a leg or chela (the carapace of Uca is too thick to 

 allow of satisfactory observation) at short time intervals, graphs 

 of the average values for each group can be plotted to show the 

 gradual dispersion of the chromatophores after the background 

 change from white to black (dotted curve) and the concentration 

 in the reverse case (full curve. Fig. 3-15«; Brown, 1950Z>). 



The first conclusive evidence that such changes in the chromato- 

 phores were controlled by a hormone was obtained by Perkins 

 (1928), who showed for the prawn, Palaemonetes, that, although 

 cutting the nerve supply to any part of the body does not interfere 

 with its colour responses to a change of background, a ligature 

 occluding the blood supply to that part stops the response, as in 

 the stick insect Carausius (§ 3.221). Release of the ligature restores 

 the response. 



Concentration of red pigment in Palaemonetes. The source and 

 action of the red-concentrating hormone that causes the response 

 to a white background has been identified in Palaemonetes by the 

 pharmacological method. In the first place it is found that, in 

 prawns and other Decapoda, except the Brachyura, removal of 

 the eyestalks destroys the background response and leads to 

 permanent dispersion of the pigment in the red chromatophores ; 

 extracts of the eyestalks can overcome this dispersion and lead to a 

 temporary concentration of the pigment. Within the eyestalk the 

 most potent source of the red-concentrating or Palaemonetes- 

 LiGHTENiNG HORMONE, PLH, is in the sinus gland. Moreover 

 eyestalk extracts from most Decapoda, except crabs, produce this 

 pigment concentration, if injected into a dark, eyestalkless prawn ; 

 PLH is therefore not specific to any particular genus (Brown, 

 1950^). 



Much contradiction in earlier work was due to the fact that 

 crude eyestalk extracts, containing PLH, were admixed with 

 varying amounts of a second type of hormone that has a darkening 

 effect on crabs. These two kinds of hormones were first separated 



