THE MUTATED GENE 259 



A beginning of such an analysis has been made for a similar 

 object, the feather areas (pterylae) of birds, which certainly 

 again present areas with a center of outlet of feather-determining 

 stuff. Within such an area, according to Holmes (1935), first 

 a row of follicles are formed simultaneously; from here the process 

 spreads laterally and medially as well as anteriorly and posteriorly 

 and might therefore be described as a determination stream. 

 Within such a field, certain regularities have been found by Juhn 

 and Fraps (1934); Juhn, Faulkner, and Gustavson (1931); and 

 Fraps and Juhn (1936). The feathers on both sides of the first 

 row may behave as mirror images in regard to structure and color; 

 their rates of growth and length of regenerated shaft have a 

 definite size increasing in an anterior-posterior direction. Fraps 

 and Juhn showed a similar relation for barb length in regeneration 

 relative to the distance of the feather from the central row and 

 were able to describe this in terms of field functions (which is 

 only a description in mathematical terminology for the same 

 things which we treated thus far as spread of stuffs). These 

 pterylae coincide partly with areas of pigmentation (Juhn), 

 and possibly the same processes are involved in their formation. 

 Since here, again, mutant genes are known controlling the 

 different features of the pattern, we may expect more information 

 from this material. 



Another attack upon this problem has been made by Goodrich 

 (1933), in fishes. We have referred to his work on Oryzias where 

 he could show that black and yellow melanophores are involved. 

 In yellow fish, the melanophores are present but not able to 

 form pigment. As the Dopa reaction shows, the oxidase is not 

 lacking, but the chromogen which is produced by the action 

 of the dominant gene. The difference between the two alleles 

 is thus one of quantity of effect. This effect takes place in the 

 individual cells; it is cell localized. As these chromatophores 

 migrate in the embryo from a center of origin, their determination 

 in regard to chromogen formation occurs probably in this early 

 stage. There is also a variegated type which requires that in 

 the early stage a pattern has already been formed of cells with 

 different powers of chromogen formation. But this pattern could 

 just as well be determined later by local conditions. No experi- 

 ments of the type of Twitty's (see page 179) have yet been 

 performed to answer this question. 



