EFFECT OF THYROID FEEDING. 279 



feather may arise from the same germinal papilla as its prede- 

 cessor, a period of inactivity of the papilla follows the withdrawal 

 from the calamus of the first and the rounding off of the inferior 

 umbilicus. A new feather is then begun, within a sheath of 

 its own. 



The rounding off of the tip of the calamus of the first feather 

 and the establishment of a discontinuity between the first feather 

 and its successor involve certain processes of differentiation 

 before the feather follicle and papilla become quiescent. One of 

 the effects of thyroid feeding in our experiments was to suppress 

 these terminal processes of differentiation by maintaining the 

 papilla in continuous, even though, it may be, reduced activity. 

 This continuity of process was revealed in continuity of structure 

 and, incidentally, settled, in this case, the question that has 

 sometimes been raised whether successive feathers from the same 

 follicle spring from the same papilla. Here the second obviously 

 did spring from the same papilla as the first. If the feathers of 

 succeeding moults arise similarly, the series, in place of a single 

 feather, would appear to be due to a rhythmic variation in the 

 velocity of critical reactions conditioning the development of the 

 papilla. This variation, normally extreme and leading to dis- 

 continuity, is essentially independent of the normal thyroid 

 secretion. The addition of thyroid to the ration under the 

 conditions of our experiments lessened the amplitude of the 

 rhythmic variation without determining the rhythm itself. 



The "quilled" first rectrices were originally noticed in our 

 thyroid-fed chicks as distal appendages to the members of the 

 second set, since they were not being shed in normal fashion. 

 Examination showed that the structure of the inferior umbilicus 

 was unusual, the proximal end of the calamus being wide open 

 and the plane of the orifice running ventrodorsally at a sharp 

 angle with the axis of the calamus. Dorsally the lip of this 

 oblique orifice was extended proximally as though the quill had 

 been cut to form a long pen point. 1 



1 In exceptional cases, as in a wing quill before us, this proximal extension is 

 not a prolongation of the median dorsal line of the calamus, but of a region perhaps 

 twenty degrees away from it. Though the median dorsal lines of the two feathers 

 do not lie in the same plane, there is no obvious structural twist to account for 

 the fact. 



