CAUSE OF PRODUCTION OF "DOWN." 167 



duced in the " down-quill " are in every way similar to those of a 

 series of defects known to be produced by malnutrition, it 

 becomes extremely probable that an insufficient food-supply is 

 he cause of the "quill " formation also. 



2. The case just cited is supplemented and strengthened by the 

 peculiar structure of certain feathers from the humeral region (of 

 one of the chicks mentioned in the experiment above) which were 

 able to continue their growth under the conditions of my experi- 

 ment. These feathers have all the appearance and texture of down, 

 excepting the presence of a very slender shaft. The barbs, how- 

 ever, are not closely set into this shaft as they are in a normally 

 grown feather, but unite with it only at wide intervals. This is 

 clearly an approximation to the conditions in the down where 

 the barbs do not unite at all. The shaft, moreover, bears barbules 

 and these again are exactly like those borne on barbs. Such a 

 feather is shown in Fig. 1. 



3. In the plumulaceous basal parts of the feathers of the chick 

 I have produced structural conditions which are in many ways 

 like those of down, and am in a position to state definitely that 

 they were produced by " starving " the bird. In Fig. 2 is shown 

 a section of a feather bearing such downy formations. 



4. A fourth line of evidence that the down — or rather its 

 basal portion — is produced under poor nutritive conditions is 

 afforded by the fact that the most emphasized of the down mal- 

 formations — the horny cylinder or quill — is to be found most 

 frequently among the altricial birds as was pointed out by Jones. 

 Jones does not state that the " quill " is the most extreme modi- 

 fication of these downy structures, but both his work and mine 

 confirm that view. 



I quote the following single paragraph which Jones writes on 

 quill-formation, and which seems to include something else quite 

 as important : 



" The progress of transition which results in a so-called 

 ' quill ' or tube differs in some important particulars from that 

 just given. In the early stages of development no difference is 

 recognizable, but at a little later stage the whole mass of inter- 

 mediate cells (Fig. 45, PI. IV., cl.in) as well as the sheath cells 

 (cl.tu) become much flattened, their nuclei elongated, and their 



