FORMATIVE COLOR FACTORS. 21 



further chemical investigation should show that what albinos lack is really 

 chromogen, as Cu6not supposed, no change in the gametic formulae here used 

 will be necessary, beyond the addition of a symbol C (chromogen) in the case 

 of colored animals. 



In the writer's opinion the only enzyme which all albinos in common lack is 

 that which acting alone produces the lowest grade of melanin pigment, namely, 

 yellow. To this supposed factor I have given the designation Y. The factor 

 B may or may not be present in an albino, but Br apparently is never lost. 

 Y then forms the step between color and no color, producing yellow when pres- 

 ent, no color being produced in its absence. This yellow is converted to brown 

 pigment by the ever-present factor Br, whose action, however, is locally inhi- 

 bited wherever yellow pigment appears as the final oxidation stage. 



2. The Bbown-Producing Coloh Factok, Br. 

 This factor, in my experience, is never wanting, though its presence is 

 often at least partly concealed by the factor B, which carries forward the 

 process of pigment formation from the brown to the black stage. In albinos, 

 also, when Y (the factor for producing yellow) is absent the factor Br, lacking 

 the material on which to act, does not function visibly. Thus it is not to be 

 wondered at that this factor Br has been omitted from the zygotic formulae of 

 most investigators and that brown is considered commonly the result of the 

 absence of black. The chemical evidence already given, however, together with 

 the fact that brown pigment is often, if not always, to be found in black animals, 

 seems to justify the use of a distinct term to designate the factor that forms 

 brown. This factor is not to be confused with the absence of the factor for 

 "black formation, which is a pure negation and can not be considered as an active 

 agent in the formation of anything. 



3. The Black-Pkoducing Colob Factor, B. 



As before stated, this factor is necessary for the production of the highest 

 stage of mammalian pigment, viz, black. In its absence, the final oxidation 

 stage is brown, so that we may speak of " animals lacking black " as brown, but 

 it is conceivable that animals might be found which also lack Br, in which case 

 the final oxidation stage would be yellow, not brown, and mere absence of black 

 would not in that case be equivalent to brown. 



If we cross, then, animals homozygous for the factor B with animals 

 entirely lacking that factor, we should expect Fi to consist entirely of heterozy- 

 gous black animals. By experiment, 589 animals have been produced in this 

 way, all of which possessed black pigment. We should expect, now, that if such 

 black animals are crossed inter se, the next generation will consist of three 

 animals possessing black pigment to one lacking it completely. When this 

 cross was made the following result was obtained: 



