212 REYNOLD A. SPAETH 



In the case of the melanophores there is no direct evidence at 

 hand for a loss or exchange of fluid during contraction. There 

 is, however, a visible and reversible colloidal aggregation of me- 

 lanin granules following a variety of physiological stimuli, all of 

 which elicit contractions in smooth muscle. Similarly, a num- 

 ber of physiological stimuli that produce a relaxation in smooth 

 muscle bring about a dispersion of the pigment granules in the 

 melanophore. Now, from the evidence cited above, we are 

 forced to assume that, during the contraction of smooth muscle 

 there is an invisible aggregation or heaping up of colloidal par- 

 ticles which becomes reversed in relaxation, i.e., when the muscle 

 relaxes, there is an increased dispersion of the in\'isible colloidal 

 particles. The aggregation of melanin granules within the melano- 

 phore must therefore he considered a visible expression of the colloidal 

 phenomenon^ that occurs, upon stimulation, in the niicro-honiogene- 

 ous colloidal content of a smooth muscle cell. 



(iKNERAL APPLICATION ()K THE CONCLl'Slo.X 



Although much of the evidence in the preceding sections has 

 been obtained from experiments with a single species of teleost, 

 Fundulus, there is no valid reason for doubting that the mechan- 

 ism of the responses in the chromatophores of crustaceans and 

 the melanophores of fish, amphibia and reptiles, is identical.^' 

 In all these cases the color-changes are brought about by an ag- 

 gregation and dispersion of pigment granules. Chun (1. c.) has 

 shown that in cephalopods the chromatophores develop from a 

 single smooth muscle cell. We may therefore say that it seems 

 highly probable that color changes in crustaceans and cephalo- 

 pods, as well as in the three groups of lower vertebrates, are 

 brought about by the physiological responses of specialized 

 smooth muscle cells. 



The physiological modification in the case of the melanophore 

 may be compared with that of the electric organ in fish where a 

 striated muscle cell becomes so far modified that the normal mo- 



'" The term 'chronuitophores' etui not 1k^ applied to llu; case of the lower 

 vertebrates until more is known of ilic ])liy.siology of the xantliophores. 



