lo Jones — Nestling Feathers. 



welche ihn bildeten. Auch findet man leicht einen einzelnen Dunenstrahl, 

 welcher sich in zwei definitive Federaste theilt. Sogar bei vollkommener 

 Verschmelzung der Dunenstrahlen und Bildung einer typischen Spule kann 

 man oft sehen, dass diese sich an ihrem tieferen Ende in vier oder fiinf 

 grosse Theile auflost, welche erst allmahlich sich zertheilen, um die definitive 

 Federaste zu bilden. Wir konnen thatsachlich jeden Ubergang wahrnehmen 

 zwischen der typischen cylindrischen Dunenspule und der direkten Fortset- 

 zung der Dunenstrahlen in eine gleiche Anzahl von definitiven Federasten" 

 (586-7). 



V. THE RELATION OF THE DOWN TO THE FIRST DEFINITIVE FEATHER. 



Klee (1886) and Davies (1889) announced that the first definitive fea- 

 ther is produced by an ingrov^th of the base of the down papilla, with the 

 cornification of the proximal end of the down. In other words, the first 

 down and its succeeding first definitive feather develop from the same cutis 

 papilla. 



Klee (p. 39) holds that the shaft of the definitive feather is continuous 

 with the shaft of the down "feather," which is clearly an error, as I shall 

 show. Davies (quotation antea 7 and 8) regarded the nestling down as a 

 structure typically complete in itself, possessing a quill and sometimes a shaft, 

 and he considered the lack of a quill as an exceptional condition. I shall at- 

 tempt to show that a shaft and a true quill are never formed, but that there 

 is always a direct continuity between the barb-vanes of the down and those 

 of the first definitive feather. 



There seems to be no detailed account of the manner in which the barb- 

 vanes of the down pass into the barb-vanes of the first definitive feather. 

 I shall therefore discuss this point at some length. 



Figures 140, 141, and 142, Plate IV, illustrate three conditions of the 

 barb-vanes after complete cornification, in the region connecting nestling 

 down with first definitive feathers. In Figure 140 the barb-vanes are united 

 into a tube surrounding the pulp cavity. In Figure 141 the barb-vane has no 

 connection with any other barb-vane. It divides into three definitive feather 

 barb-vanes. In Figure 142 the same condition is seen, but barbules are pres- 

 ent along the whole course of the barb-vane. 



Figure 140 represents the condition where a horny cylinder is formed at 

 the proximal end of the down into which the down barb-vanes disappear 

 proximally and out of which the definitive feather barb-vanes emerge. This 

 horny cylinder is the so-called "quill" of the down. I have found this to be 

 a rare condition in all birds except the Picarian and Passerine birds, where it 

 is more often found than elsewhere (PI. VIII, Figs, 124 to 136, and 138). 



The condition represented by Figure 141 is a common one. Barbules 

 are often lacking for a short distance — usually less than a millimeter — distal 

 to the branching of the barb-vane into the definitive feather barb-vanes, when 

 branching occurs; but barbules are often indicated by irregularities upon the 



