354 OSCAR RIDDLE. 



sally related to the production of the fault-bars, and of the reduced 

 pigmentation (light fundamental bars) soon to be considered. This 

 is a possibility which should be considered in view of the fact that 

 a reduction of temperature of 0.7 ° C. will, according to the rule 

 of van't Hoff, reduce the speed of chemical reactions by one fif- 

 teenth. Furthermore, it has lately been stated by Tower i that 

 in insects (where most experimental work on coloration has been 

 done) " the most potent factors in the modification of color are 

 temperature and moisture" (p. 214). I do not believe, however, 

 that temperature is an important factor in the cases we are now 

 considering. The various, parts of the growing region of the 

 feather undoubtedly have the same temperature and yet they grow 

 unequally; this unequal growth of parts such as pigment, barbs 

 and barbules under fault-bar producing conditions is really our 

 whole problem, and has been proved to be caused by a factor — 

 a reduced nutrition — which affects these parts to a different 

 extent. 



Relation of Pigment to Fault-Bars and Nutrition. 



The position of the pigment cells between the cylinder-cell 

 layer and the barbule-forming cells has already been pointed out. 

 It will be noted, too, that their processes reach the barbule cells 

 and are, in fact, rather closely applied to them at the time the 

 latter secure their pigment. In just the same way that a lack of 

 nutrition checks the production of barbule-forming cells, it re- 

 duces the amount of pigment formed and taken up by the bar- 

 bule cells. 



It seems very certain that this last statement is true, for a 

 macroscopic examination of the completed feather shows a defi- 

 ciency of color in this region ; and some parts of the cross-sec- 

 tions of such areas of adult feathers show plainly upon micro- 

 scopic examination that less pigment was produced there. It is, 

 however, very difficult to show by microscopic means that in the 

 living, growing feather-germs less pigment is being produced, 

 because of the fact that the pigment cell processes are never able 

 to even approximately empty themselves, and always appear 



1 Tower, W. L., " Evolution in Chrysomelid Beetles of the Genus Leptinotarsa, u 

 Carnegie Institution, 1906. 



