28O HARRY BEAL TORREY AND BENJAMIN HORNING. 



At the tip this penlike extension was continuous with the 

 rhachis of the proximal feather. Elsewhere the lips of the 

 orifice, including the edges of this dorsal extension, were con- 

 tinuous with the distal ends of about fifteen pairs of barbs of the 

 proximal feather. The original orientation of the barbs, which 

 had differentiated from long spiral ridges in the wall of the 

 cylindrical feather germ, was thus preserved, and the orifice 

 correspondingly obscured. 



The falling of the distal feather was accomplished by rupture 

 of these connections. As a result, the distal end of the second 

 rectrix presented a frayed appearance. This came to be recog- 

 nized as a characteristic of the second rectrices of thyroid-fed 

 birds, in sharp contrast with the smooth rounded ends of the 

 vanes of normal birds. 



4- 



What had been said regarding the effect of thyroid feeding on 

 the development of the first rectrices and the first moult applies 

 also to wing quills, though much less frequently and with slight 

 unessential differences in detail; such, for instance, as the 

 obliquity of the umbilical orifice. Contour feathers are not 

 affected, according to our observations. 



The facts may be briefly summarized. Thyroid feeding 

 inhibits: (i) the differentiation of proximal barbs and barbules, 

 thus shortening the vane without proportionately shortening the 

 calamus; the proximal region of the vane passing into the after 

 shaft normally without traumatic defect; (2) the usual differen- 

 tiation of the inferior umbilicus; (3) the withdrawal of the 

 feather pulp from the calamus, and the normal inactive period 

 of the pulp. Continuity between first and second rectrices is 

 the result. 



Thyroid feeding also (4) accelerates the differentiation and 

 eruption of the second rectrices. This acceleration was observed 

 first in Rhode Island Red males (Torrey and Horning, 1922). 

 That it occurs in Brown Leghorns is shown by the facts in the 

 next paragraph. The precocious development of the second 

 rectrices correspondingly limits the maximum growth period of 

 the first and probably accounts, at least in part, for the reduced 

 length of the latter after thyroid. 



