THYROID AND GONAD IN PI. IMAGE. 223 



II. 



This darkening of the plumage, to which we originally called 

 attention in a preliminary note (1923, i) was also observed by 

 Cole and Reid (1924). The plate accompanying their paper 

 illustrates very well certain typical color changes produced in 

 the feathers of Brown Leghorns by thyroid feeding. With a 

 view to repeating our observations (1922) on the appearance of 

 plumage of the female type on young Rhode Island Red males as 

 a result of thyroid feeding, these authors found in thyroid fed 

 Brown Leghorn cockerels that had assumed adult plumage 

 changes in feather form and structure that are probably typical 

 responses of poultry in general, but which Crew and Huxley 

 (1923), on a similar errand, failed to note. 



What is true of Brown Leghorns in this connection is also 

 true of Campines, Barred Plymouth Rocks and Rhode Island 

 Reds. White Leghorns show similar changes in feather form and 

 structure, though not in color. This exception is likely to 

 prove the rule among other dominant whites as well. There 

 is a definite correlation between structure and color in these 

 experiments, however, which brings both pigmented and non- 

 pigmented races into a single category. 



While these different races responded thus similarly to thyroid 

 feeding, they did not do so with equal intensity. Of our pig- 

 mented races, the Brown Leghorn was most affected, the Rhode 

 Island Red least. These differences in reaction associated with 

 differences of race, however, were not conspicuous, nor are they 

 so interesting, as those associated with differences in sex. 



The results of thyroid feeding are peculiarly conspicuous in 

 Brown Leghorn males, especially in neck hackles, shoulders, 

 backs and saddles, all feathers that are brilliantly colored in 

 shades of red and show marked sexual dimorphism. In the 

 male, these feathers are distinguished by a lacy border of naked 

 barbs (Fig. i). On a thyroid diet, the latter tend to clothe 

 themselves to their tips with extensions of the normal two rows 

 of barbules (Fig. 2). As the barbules thus extend toward the 

 edge of the feather, so also does the melanin pigment; for, as a 

 rule, this pigment is carried by the barbules and limited by their 

 distribution. As a result of thyroid feeding, the zone of barbules 



