74 KINETIC HORMONES — I 



"plasmosol phase" out into the tips of the cell branches. Dispersal 

 may then be a re-stretching of these proteins on solation, and not 

 a matter of Brownian movement. High pressures (up to 8000 

 Ib/in^) can be shown to cause solation, and to inhibit the concen- 

 tration of the granules. This is on a par with amoeboid movement 

 and particularly with muscle contraction ; but the problem is still 

 unsolved of whether either nerve stimulation or hormone action 

 can affect the state of the chromatophore proteins in the same way 

 as in the muscles, or whether changes in osmotic pressure 

 (Abramowitz and Abramowitz, 1938) or in permeability of the cell 

 membrane play a part. As a rule chromatophores react much more 

 slowly than muscles ; but there is a great difference in the reaction 

 speeds of the similar-looking chromatophores of Amphibia, 

 Reptilia and various Crustacea. This may be a question of the 

 strength of stimulation. 



The effectors with movable pigment fall into three types. 



(1) The epidermal cells of certain Insecta are relatively un- 

 specialized, except for the presence of pigment granules, which 

 may be of more than one colour and may move in different direc- 

 tions (Fig. 3-10, § 3.221), 



(2) The retinal pigment cells of certain Crustacea and Insecta 

 occur in two positions, proximal and distal, round the ommatidia 

 of the compound eyes (Fig. 3-12, § 3.222), and contain moving 

 granules of black pigment. Movement of reflecting, or white 

 pigment, granules in cells round the retinulae also occurs. 



(3) The branched and specialized chromatophores of Crustacea 

 and some other invertebrates, as well as of lower Vertebrata, may 

 be epidermal, but are usually mesodermal, and may contain more 

 than one pigment; but if so, each pigment remains in its own 

 branch of the cell (Plate 3-1, § 3.223). 



3.221 Pigment movement in epidermal cells 



Insecta. The cell-boundaries are reputed to disappear between 

 moults, and the pigment granules can then migrate to an extent 

 comparable with that in chromatophores. In green specimens of 

 the stick insect, CarausiuSy the colour change is obscured by 

 stationary yellow-green pigment; but in brown specimens the 

 red pigment moves parallel to the surface of the epidermis, and the 



