EFFECT OF THYROID FEEDING. 277 



The condition of the rectrices in all groups on May 18, 52 

 days after hatching, is shown in Table I. 



The shortening of the vane characteristic of the "quilled" 

 feathers was due to a failure of a considerable number of barbs 

 to differentiate proximally. There was no sign of a loss of 

 barbs previously formed and broken off. The vane of each 

 "quilled" feather, though abnormally proportioned as already 

 indicated, ended below in an aftershaft of barbules surrounding 

 the superior umbilicus, with no sign that there had ever been 

 barbs proximal to this point. 



As is well known, the barbs of a feather arise from the so- 

 called intermediate cells of the feather germ epidermis, these 

 intermediate cells differentiating in the growing zone into parallel 

 ridges, which, as the feather germ elongates, run a longitudinally 

 oblique course about the dermal core. These ridges differentiate 

 beginning with their distal ends into the barbs and barbules of 

 the definitive feather and are continually renewed from below. 



Now the effect of thyroid feeding was to abbreviate the period 

 of ridge formation in the growing zone, and as a consequence to 

 suppress prematurely in that region the processes of differentia- 

 tion on which barb formation normally depends, without sup- 

 pressing, however, the processes on which the lengthening of the 

 calamus depends. It is true that the total length of each of the 

 "quilled" feathers thus produced was somewhat less than normal, 

 but that this was not to be attributed to a direct inhibitory 

 effect of the thyroid upon the growth of the feather germ, but 

 rather to a precocious development of the second rectrices, will 

 appear, we believe, from facts to be given below. It may be 

 said here, however, that while the suppression of barb ridge 

 differentiation occurred prematurely after thyroid, in rare cases 

 these ridges might appear again prematurely, after thyroid, about 

 the inferior umbilicus of the first rectrix, foreshadowing the 

 base-to-tip connection of first and second rectrices, to be de- 

 scribed in the next section. 



3- 



In size, shape and coloration details of which do not here 

 concern us the first rectrices, belonging to the first few weeks 

 of the chick's life after hatching, differ distinctly from the 

 second, which are adult in type. 



