226 



The Journal of Heredity 



Skin color in ji;cnL'ral corresponds 

 roughly with hair color. The pij^ments 

 in the eyes arc like those in the skin and 

 fur, but the ajipearance is j^enerall\' much 

 modified by structural effects. In dilute 

 human eyes, for example, the ajspearance 

 is blue, although the pigment is sepia 

 brown. In the rodents the reflection 

 from the back of the retina gives a red 

 color when the pigment is reduced, which 

 in the complete absence of pigment 

 becomes pink as in albinos. 



CHEMISTRY' OF .MIiL\.M[M 



A larg3 amount of work in the last 

 twenty years has firmly established the 

 hypothesis that mjlanin is an oxidation 

 l^roduct of tyrosin or related products 

 of protein metabolism. En/.ymes have 

 been extracted in a great many animals 

 and plants, which have the power of 

 oxidizing tyrosin and related substances 

 to dark brown ijigments closely re- 

 sembling the natural melanins. vStudies 

 of Hooker,'* on cultures in vitro of 

 frog mtbenchyme indicate that the 

 pignient granules are formed in the 

 cytoi)lasm iminediately surrounding the 

 nucleus, i)resumably under the influence 

 of oxidizing enzymes secreted by the 

 latter. 



The nature of the differences between 

 colors is still far from clear. Onslow* 

 could find no chemical differences be- 

 tween pigments extracted from black 

 and yellow rabbits and considers that 

 they differ merely quantitatively. Most 

 others do find chemical differences as 

 well as physical ones. Black pigment 

 seems always to be granular, while red 

 may be either granular or diffuse. 

 Lloyd-Jones* found only granules in 

 both intense and dilute black pigeons, 

 and found red granules in red pigec^is, 

 but merely a diffuse yellow in the 

 dilute yellows. The Davenports^ foimd 

 granules in brown and l)lack human 

 hair, but merely a diffuse color in auburn 

 hair. 



There is a distinct difference in 

 solubility between the dark colors and 

 yellow. Miss Durham'* found that 

 yellow granules in yellow mice dissolve 

 quickly in potash, brown less quickly, 

 and black not at all. Gortner^ was 

 able to separate two pigments in black 

 wool — one easily soluble in very dilute 

 alkali and also soluble in dilute acids, 

 while the other dissoh'ed only slowly in 

 alkali and not at all in dilute acids. 

 This, of course, is a chemical difference. 

 The acid-soluble type was of protein 

 nature. He considered it to be a diffuse 

 coloration of the keratin structure, 

 while he identified the insoluble type 

 with the granules. In red human hair 

 he f<3und only the acid-soluble type. In 

 brown horse hair and black wool he 

 found both, while in brown and black 

 human hair, black rabbit hair and black 

 feathers of domestic j^oultry and crows 

 he found only the insoluble type. His 

 two types evidently correspond more or 

 less closely with yellow and black 

 pigment. This dift'erence in solubility 

 does not necessarily indicate that yellow 

 and black are produced from differ- 

 ent chromogens. Indeed, Gortner has 

 shown that the insoluble type may be 

 produced from the soluble by treatment 

 with strong alkali. In a brief paper in 

 1912, however, Gortner^" reports on a 

 more fundamental difference between 

 the insoluble melanin from black 

 feathers, black rabbit hair and brown 

 horse hair, and the soluble melanin from 

 black wool and brown horse hair. 

 The former yielded some 3% ash con- 

 sisting largely of iron oxide, while the 

 latter yielded little or no ash. This 

 seems to demonstrate a difference in 

 chromogens in at least these cases. 

 But even here black pigment may con- 

 sist of a mixture of melanins containing 

 iron and melanins which do not, derived 

 l)erha])s from the same chromogens 

 as those which under other conditions 

 produce yellow. 



* Hooker, I)., 191.S. Anal. Rcc, 9: 39,V 

 'Onslow, H., 1915. Proc. Rov. Soc, B-89: 36. 



« Llovd-Joncs, O.. 191.S. Jour. Exper. ZooL, 18: 45.?. 



^ Davenport, C. B. and G. C. 1909. Amer. Nal., ■^^: 19,?. 



* Durham, F. M., 1904. Proc. Rov. .Soc. London, 74: 310. 

 "Gortner, R. A., 1911. Biochem. Bull., 1: 207. 

 "Gortner, R. A., 1912. Proc. Soc. lixp. Biol, and Mrd., 9:3. 



