THYROID AND GONAD IX PLUMAGE. 



225 



may extend beyond the zone of melanin pigmentation; but the 

 reverse does not occur. In control birds, the reverse does occur. 

 This is especially clear in the shoulder feathers of Barred 

 Plymouth Rocks, in which dark bands pass distally across the 

 naked barbs (Fig. 3). 



FIG. 3. Representative shoulder feathers from an adult Barred P. R. male, 

 showing barring across naked barbs. 



This tendency of barbules and melanin pigmentation to extend 

 together over the feathers of thyroid fed birds is especially 

 marked by reason of the fact that the zone of barbules in male 

 birds possesses a typically notched or wavy contour with which 

 the outer edge of melanin pigment distribution perfectly coincides. 

 It is difficult to avoid the conclusion that the effective stimuli 

 for barbule and for melanin formation in these feathers are one 

 and the same. There is reason, however, for believing that this 

 is not an adequate summary of the situation. 



Everything that has been said about the male bird applies 

 as well to the capon. In the latter, such decorative feathers 



