68 INHERITANCE IN GUINEA-PIGS. 



certain oxidizing enzymes on tyrosin and related aromatic compounds. 

 Tyrosin is an important constituent of protein molecules and there is 

 much reason to believe that tyrosin and related substances are the 

 chromogens from which the natural melanins are formed. Tyrosinase, 

 an enzyme, which can oxidize tyrosin to dark substances resembling 

 melanins, has been found very widely among animals, including the 

 skins of mammals, as will be discussed later. 



There have been many theories on the mode of origin of pigment in 

 the cells. Early observations indicated that melanin was directly 

 extruded from the nucleus. Recent studies by Hooker (1915) on in 

 vitro cultures of mesenchyme and epithelium of the frog indicate that 

 melanin granules form in the cytoplasm but at the point of known 

 greatest efficiency of the nucleus as an oxidizing agent. Thus, prob- 

 ably chromogen (tyrosin or derivatives) is in the cytoplasm, while 

 oxidizing enzymes are given off by the nucleus. 



The color white in the fur of mammals is due to the absence of 

 pigment. The theory of a white melanin seems effectively disproved 

 (Gortner, 1910). A priori, the presence or absence of pigment might 

 be conceived as due either to a deficiency of chromogen or of enzyme. 

 In line with the first view, Gortner (1911) found that the pattern hi 

 the elytra of potato beetles is due to a deficiency of chromogen. Fur- 

 ther, Cuenot (1903, 1904), in the first attempt to correlate the facts of 

 Mendelian inheritance with the physiology of pigment, suggested pro- 

 visionally that albinos lack the power of producing chromogen, while 

 the different colors which he demonstrated could be transmitted 

 through albinos depend on specific enzymes. On the other hand, 

 recent observations by Onslow (1915) demonstrate that absence of 

 pigment in widely different cases in mammals depends on enzyme differ- 

 ences. He found peroxidases in the skins of gray, black, blue, and 

 brown rabbits, which produce a black pigment from tyrosin in the 

 presence of hydrogen peroxide. In the skins of albino rabbits and mice 

 and in the white part of the Dutch pattern in rabbits, all recessive 

 whites, he was unable to demonstrate a peroxidase, although there was 

 nothing present which prevented the oxidation of tyrosin to a black 

 pigment when tyrosinase was added. In the white of the English 

 rabbit, a dominant white, he did find an anti-tyrosinase. 



Finally, there is strong genetic evidence that albinism in guinea-pigs 

 is not due to absence of chromogen. A diminution in quantity of 

 chromogen should bring about the same diminution in quantity of all 

 pigments, regardless of quality. But in red-eyed guinea-pigs (which 

 we may consider as incomplete albinos, as they have an allelomorph 

 of albinism C r ) no yellow develops, leaving white areas where factors 

 of group 2 determine yellow differentiation, but there may be nearly 

 as much black as in normal guinea-pigs. Indeed, in the albino guinea- 

 pigs and Himalayan rabbits, there is no yellow, but some black. 



