§ 3.223 EFFECTORS WITH MOVABLE PIGMENT GRANULES 91 



these are precursor substances, rather than the natural hormone, 

 since they often have mixed effects or need to be "activated" in 

 some way, such as boihng or extraction in alcohol, before becoming 

 effective (Figs. 3-15-17). 



Dispersal of red pigment in Palaemonetes. The red-dispersing or 

 Prt/«^mo;z^^^5-DARKENiNG-HORMONE, PDH, that acts antagonistic- 

 ally to PLH, has been difficult to locate because it requires a test 

 animal that is pale, yet contains no source of an overriding, 

 concentrating hormone. This has been achieved by starting with 

 eyestalkless specimens of Palaemonetes in which the red pigment 

 is fully dispersed (index 5); then at time (Fig. 3-17) an extract 

 containing PLH from either sinus gland or tritocerebral commis- 

 sures is injected, and pigment concentration becomes almost 

 complete in 15 min (index 2 or even 1.5). After this, the effect 

 wears off slowly as the hormone is destroyed in the tissues and 

 the chromatophores gradually return towards full dispersion. The 

 rate of this return is unaffected by an injection of sea water at 

 30 min (Fig. 3-17, curve i). If, instead of sea water, an injection of 

 an extract of the darkening hormone, PDH, from either abdomi- 

 nal ganglia or circumoesophageal connectives is given, the rate of 

 dispersion is greatly increased (index 4 is reached in 15 min), and 

 may continue until full dispersion is achieved, within an hour 

 from the start of the experiment (Fig. 3-17, curve ii). This shows 

 conclusively the presence of PDH in the extract, whereas testing 

 this extract on eyestalkless animals which had not been pre- 

 treated and were therefore dark, though consistent with this 

 interpretation, does not by itself prove the activity of the extract 

 (Fig. 3-17, curve iii). If PDH is injected into normal, eyed animals 

 with pigment concentrated in the light, only a slight dispersion is 

 produced and this is quickly followed by a return to full concen- 

 tration, showing that the action of PDH is unable to overcome the 

 naturally secreted PLH (Fig. 3-17, curve iv). 



To allow for the natural reversal of the chromatophore response 

 within a relatively short time, a further elaboration of the two- 

 hormone hypothesis has been postulated, namely, that in response 

 to a black-to-white background change, a large amount of PLH is 

 discharged suddenly into the blood, but that "as adaptation 

 becomes complete there would be a reduction of the hormone 



