MORGAN, COAT COLORS IN MICE 109 



seen in many animals in which the tip of the hair or feather or scale is 

 brightly colored and the base dark or black. Obviously the peculiar fea- 

 ture of such structures is the stratification of the color bands. The cells 

 first formed in the follicle must produce one color, then another, and later 

 a third. The possibility of producing all three must be present at the 

 beginning, and a "morphogenetic factor" of some sort determines the 

 activity that leads in turn to the formation of one after the other of these 

 colors. It must be at once granted that we know no more of the nature 

 of such a factor than we do of the differentiation that appears in the 

 development of any of the characters of the embryo. To ascribe it to a 

 "ticking factor" is no more than to describe the phenomenon in general 

 terms, since we know nothing of the method of action of such a factor. 

 The fact that the ticking factor may be carried by the yellow mice that 

 also carry black or chocolate (or both) and yet not come to expression is 

 one of the peculiar features of the yellow inheritance. 



That the development of the ticked coat is a specific element in heredity 

 can not be doubted, and, admitting the purely symbolic nature of its 

 representation, I see no objection to calling it in the ordinary sense a unit 

 character. 5 It is interesting to see how it has been treated in Mendelian 

 literature. 



Cuenot uses the letter G as a symbol for gray and treats it as allelo- 

 morphic to the other colors — yellow, black or chocolate. Its presence pro- 

 duces a gray mouse, which in his formulas is never treated on the presence 

 and absence theory, but always as allelomorphic to yellow or black or 

 chocolate. Yet for its expression the presence of all these three pigments 

 must occur. It is in fact a bundle of these three pigments with the added 

 condition of their stratification. There is obviously an obscurity here, 

 unless the group of colors that stand for ticking is inherited as a unit in 

 allelomorphic relation to either of the three colors considered by them- 

 selves. 



Bateson's symbolism for gray in mice is G B Ch C, to which yellow 

 must, I believe, be added, giving Y G B Ch C. Miss Durham omits the C 

 unit and makes Ch its equivalent, or at least suggests such a possibility. 

 In these formula?, G is called the gray determiner which is equivalent to 

 ticking. Bateson adds, "The important question, what the effect of the 

 gray determiner, for example, actually is, remains undecided." Bateson 

 represents the black mouse as C g B Ch and chocolate as C g b Ch. In 

 these formula; small g and b stand for the absence of the gray and the 

 black determiners. 



5 This statement leaves open the nature of the factor that produces such a "unit 

 character;" 



