EPIDERMAL STRUCTURES 



101 



point dorsally, e.g. the regenerating saddle feathers of capons 

 grow more slowly than the breast feathers. A similar gradient 

 exists in the breast, but not in the saddle. 



Two further points demand special attention : one is a quite 

 new one, of obviously first-class physiological importance — 

 viz. that in all feathers low rate of growth is associated with 

 a low threshold of sensitivity to the sex-hormone, as our 

 authors have demonstrated by means of an ingenious series 

 of experiments. Accordingly, saddle feathers respond by 



Fig. 56. — Gradients in feather-growth in fowls ; constructed from the data 



of Juhn, Faulkner and Gustavsen, 1931. The ordinates represent rates of 



feather-growth per day, in mm. 



In A, the abscissa axis represents the anteroposterior axis of the breast region in a capon (0, points 

 for single feathers ; ©, means). In B and C, it represents the gradient downwards from the mid- 

 ventral line (in B, in both anterior and posterior breast regions of a cock ; in C in the anterior breast 

 region of a capon). In D, the rates of growth of anterior and posterior regions of the breast of a 

 cock and a hen are compared. 



feminization to small doses of female hormone which have 

 no effect upon breast feathers : or again, a single injection 

 of female hormone capable of inducing a large feminized 

 pattern on the slowly regenerating feathers of the anterior 

 breast produce smaller bars of feminized pattern on the more 

 rapidly-growing feathers of the posterior breast. (And see 

 p. 260.) 



The other point is a confirmation from this quite new 

 quarter of the principle we found to hold in crustacean 



