284 HARRY BEAL TORREY AND BENJAMIN HORNING. 



fault bars, being represented by actual defects in barbule forma- 

 tion. Toward the base of this "appendage," as the vane 

 narrows, they become more and more closely spaced. When 

 the vane broadens out again, beyond this narrow neck,, these 

 lines are less closely but very regularly spaced. Assuming that 

 the distance between successive lines is a measure of the daily 

 growth of the feather, the setting off of distal appendage or lobe 

 from the rest of the feather was correlated with a period of 

 diminished growth velocity due to a cause not definitely known 

 in this case, but very possibly a nutritional deficiency covering 

 about fifteen days. 



Riddle (iQoSa) interpreted in a similar way the relation be- 

 tween down and succeeding definitive feathers. Jones (1907) 

 had shown that the first down and its succeeding definitive 

 feather are produced by one continuous growth, the former being 

 the plumulaceous tip of the latter. But the down feather is 

 differentiated from the pennaceous feather, with which it is 

 continuous, by a short quill, which Riddle (1908) regarded as a 

 defective region due to the same general cause responsible for 

 fault bars, namely, inadequacy of nutrition. True quills also, 

 Riddle thought, resulted from a diminished food supply. 



There are, then, certain points of resemblances between these 

 phenomena which, whether normal or abnormal, have been 

 interpreted as results of variations in the food supply, and the 

 phenomena we have described as results of thyroid feeding. 

 There are, however, certain significant points of difference. 



Thyroid feeding shortens the vane of the first rectrix in chicks 

 and increases the proportionate length of the calamus by the 

 suppression of proximal barbs and their associated barbules. 

 This resembles effects attributed to poor nutrition. 



Thyroid feeding tends to maintain continuity between first 

 and second generations of rectrices. We have no evidence that 

 poor nutrition does this. 



Thyroid feeding tends to accelerate the differentiation and 

 eruption of the second rectrices. Poor nutrition, on the con- 

 trary, tends to retard these processes. 



It appears then that these results of thyroid feeding are of 

 two opposing sorts, only one of which is comparable with the 



