DISTRIBUTIVE COLOR FACTORS. 27 



rectly. However, I feel sure that errors of this sort have not occurred since 

 1909, and the figures given above may be considered as a correct representation 

 of the observed facts. For the most part the behavior of density and dilution 

 is clearly that of a pair of allelomorphic unit characters, and they may therefore 

 be treated as such. Thus dilute animals crossed inter se give, as expected, only 

 dilute young, 603 such animals having been produced. 



5. The Distributive Factor, P. 



As before stated, this factor affects brown and black pigment, but does not 

 affect the amount of yellow present. The modification of this factor giving 

 rise to the "pink-eyed " series of colored mice, reduces the amount of black and 

 brown pigment to a very great extent. The reduction affects hair, skin, and 

 eyes. Miss Durham (1908) has pointed out the fact that the eyes, as the hair, 

 are not entirely devoid of pigment. In examining with the microscope sections 

 of the eye of a "pink-eyed" colored mouse, she found traces of pigment present, 

 but in such- minute quantities as to make any qualitative analysis impossible. 

 The writer has examined under the microscope crushed eyes of the brown, 

 dilute brown, pink-eyed brown, and pink-eyed dilute brown varieties and has 

 thus been able to trace the steps in the reduction of pigment in these four vari- 

 eties with fair success. The eyes were prepared by dehydrating and clearing 

 in xylol. The lens was then removed and the iris and retina spread on the 

 slide and mounted in balsam. When thus prepared, the following conditions 

 were observed: 



(1) In intense-brown animals, the iris is more heavily pigmented than the 

 retina, but both are thickly pigmented with brown only, no yellow. 



(2) In dilute-brown animals, there is reduction of the total amount of pig- 

 ment, but the retina shows the effects of the reduction more than does the iris. 

 The pigment is brown only, no yellow. 



(3) In pink-eyed intense-brown animals there is very little pigment in the 

 retina. The iris is moderately pigmented, but much less so than in the dilute- 

 brown form. The pigment is brown only. 



(4) In pink-eyed dilute-brown animals the retina is apparently unpig- 

 mented, but the iris still contains a small amount of brown pigment. No 

 yellow pigment is present. 



We may now turn to the experimental results obtained in crosses between 

 dark-eyed animals (PP) and pink-eyed animals (pp). In this case Fi consists 

 entirely of dark-eyed animals (Pp), 1,085 individuals having been thus obtained, 

 all dark-eyed and to external appearances indistinguishable from homogyzous 

 dark-eyed animals. These F x heterozygotes crossed inter se have given the 

 following result : 



