COLOR MUTATIONS IN MICE OP PEROMYSCUS 293 



are nearly or quite lacking in some yellows, and that, when 

 present, they are probably confined to the darker, middorsal 

 region of the body. It is not improbable, also, that the yellow 

 pigment of the agouti hairs is more abundant or more highly 

 concentrated in this color variety. This is yet more likely in 

 the case of the 'b' strain to be described next. 



As also stated in earlier papers, the ventral pelage of the 'yel- 

 lows' is more intensel}^ white than that of the 'wild type.' This 

 is due to the greater length of the terminal pigmentless zone of 

 the hairs on the under surface of the body. Indeed, in the mid- 

 ventral line, the basal 'plumbeous' zone of the individual hairs 

 is commonly quite lacking, the pelage being entirely white. 

 There appears to be no reduction, however, in the depth of 

 pigmentation of this basal zone throughout the body, nor do 

 the tail, feet, ears, eyes, etc., appear to show any diminution 

 in the amount of pigment normally present. 



jNIice of this strain differ considerably from one another in 

 shade. In some specimens the entire pelage, or certain areas, 

 exhibits a decidedly 'dusky' hue, owing, apparently, to the pres- 

 ence of a larger proportion of the all-black hairs or of hairs heavily 

 tipped with black. Likewise, the richness of the color varies 

 very much, the brighter specimens displaying considerable 

 orange-yellow, the duller ones having a 'washed-out' appearance, 

 suggesting that of jute or some similar vegetable fiber. These 

 differences are probably due to the amount of the quality of the 

 yellow pigment in the individual hairs, though no careful micro- 

 scopic examination has thus far been made.^ On the whole, 

 the mice of the 'a' strain are of a decidedly less rich hue than are 

 those of the '6' strain next to be described. 



Only eight adult skins of the former strain are available for 

 color tests with the tint-photometer. These give the following 

 mean and extreme readings, in terms of the three 'primary' 

 colors of the color screens. The computed proportions of black, 



^ Studies of the quality and distribution of hair pigment, in relation to sub- 

 species, color mutation, and behavior in hybridization, are being undertaken in 

 this laboratory by Mr. R. R. Huestis. The matter has already been dealt with, 

 to some extent, by Sumner ('18). 



