24 THE INHERITANCE OF COLOR IN MICE. 



quantity of pigment, we might expect that they would show more fluctuations 

 than do the formative or quahtative factors Y, Br, and B; and such is the case, 

 as will presently be more fully described. 



4. The Distributive Factoh, D. 



We have stated that this factor is responsible for the degree of density of 

 yellow, brown, and black pigmentation seen in the common wild house-mouse. 

 The presence of this factor is allelomorphic to a modification of density of pig- 

 mentation which we may call " dilute " pigmentation and designate by d. This 

 dilute type of pigmentation was recognized by Bateson (1903) in mice. At 

 that time he stated that it consisted of a reduced number of pigment granules, 

 affecting both the cortex and medulla of the hair. To this the writer may add 

 that the pigmentation of the skin is visibly lessened in " dilute " forms, and that 

 the eyes are also affected. When the eye of a "dilute" mouse is examined 

 under the microscope, reduction in the amount of pigment, as compared with 

 the densely pigmented forms, is seen to exist. This reduction occurs in both 

 retina and iris, but is more extensive in the former locality than in the latter. 



The dilute forms of rabbits, as compared with the intense forms, show, on 

 microscopic examination of the hair, a reduction in number of the pigment gran- 

 ules in the cortex of the hair. The dilution in this case seems transferable to 

 hair of any color and is therefore comparable to dilution in mice, as it is also 

 in superficial resemblance, having a streaky and washed-out appearance to the 

 unaided eye. 



In guinea-pigs the condition seems somewhat different. The problem 

 there does not appear to have been worked out with especial reference to dilu- 

 tion. There dilute forms do not appear streaky, although the cream type of 

 reduction seems either directly or indirectly transferable to black and brown. 

 It seems, however, doubtful whether a true dilution exists in guinea-pigs. There 

 is no evidence showing a modification of the intense colors, which modification 

 behaves as a unit character; and it seems probable, from the recent occurrence 

 of a pink-eyed colored guinea-pig (Castle, 19126), that the reduction of pigment 

 seen in guinea-pigs is due to a modification approaching that seen in pink-eyed 

 mice, rather than the dilute modification. 



The "dilute" modification affects all three pigments (yellow, brown, and 

 black), wherever they may occur. Castle and the writer (1909) recognized 

 this in mice in the " non-yellow " varieties, including those possessing the agouti 

 factor. Plate (1910) recognizes a dilute series of color varieties, though using 

 a shghtly different nomenclature to designate them. Morgan (1911a) inde- 

 pendently puts on record the occurrence of dilute animals with the agouti 

 pattern. Miss Durham (1911) also recognizes the existence of a dilute type 

 of every color variety. Previous to that time, however, she had not recog- 

 nized the relation of the dilute agouti animals to the dilute ' ' non-agouti ' ' forms. 



Entirely distinct from this modification of the factor for density of pig- 

 mentation, which produces dilute forms, is the fluctuation of the degree of 

 density itself. Though "density" and "dilution" act as a pair of allelomor- 



