FORMATIVE COLOR FACTORS. 19 



LOCATION OF PIGMENT. 



In wild mammals all three pigments (yellow, brown, and black) commonly 

 occur in the same hair. In many domesticated or '"fancy" varieties, one or 

 the other pigment prevails almost, if not quite, to the exclusion of the others; 

 thus yellow, brown, and black varieties arise. Brown varieties (completely 

 lacking black) occur in certain mammals (e. g., mice, rabbits, guinea-pigs, 

 and dogs). Theoretically they would seem to be possible in all. 



Pigment appears in mice in three general regions. These are the eyes, the 

 skin, and the hair. The pigment occurs in granules. 



In the eyes microscopic examination has shown that the pigment granules 

 are located in both the retina and the iris. Of these two localities the iris 

 seems the more permanent pigment-producing center; for when reduction in 

 the amount of pigment occurs the retina seems the more easily and extensively 

 affected. 



In the skin the pigment is located only a short distance from the surface 

 in the malpighian layer. In contradistinction to the eyes, which are visibly 

 pigmented at birth, the skin apparently becomes pigmented only after the 

 animal is several days old; this is largely because the hairs, in which much of 

 the pigment lies, are not formed at birth. The pigment granules of the hair 

 may be clearly observed under the microscope. Bateson (1903), among other 

 investigators, has determined their location with considerable accuracy and 

 finds that they are situated in the cortex or outer sheath of the hair, and also 

 in the interior of the hair in the proximal walls of the medullary spaces. 



To recapitulate, we find three general pigment-producing regions in mice: 

 thej-y e. the skin, and the hair. The pigments (melanins) produced are limited 

 to three: yellow, brown, and black — which we consider to be caused by the 

 action (jointly or severally) of the factors Y, Br, and B. Domesticated or 

 "fancy" varieties of mice appear to differ from the wild in the partial or com- 

 plete exclusion of one or more of the pigments. Such exclusion may be the 

 result of (1) the action of a distributive factor, or (2) the loss of the formative 

 factor necessary to the production of a particular pigment. 



FORMATIVE COLOR FACTORS. 



1. The General Color Factor, Y. 



It has long been recognized by investigators that, in mice, pigmented vari- 

 eties differ from albinos by a single mendelian unit-character. Thus in crosses 

 between homozygous self-pigmented and albino animals we find that the first 

 generation (F^ consists entirely of pigmented animals. If these first-genera- 

 tion hybrids be then crossed inter se we should expect in F-> that the ratio 3 

 pigmented young to 1 albino would be approximated. The experimental results 

 agree well with this expectation and we have as a result of crossing Fi inter se: 



