STRONG: DEVELOPMENT OF COLOR IN DEFINITIVE FEATHER. 161 



6. Cornijication and Withdrawal of the Feather. 



With cornification, the barb coi'tex differentiates from the surround- 

 ius; tissue and the outhnes of individual cells become less and less evi- 

 dent, until, finally, in the fully cornified barb there is little or no 

 evidence of its former cellular nature. The nuclei of the barbule cells 

 shrink, and the last seen of them is a small glistening mass of shrivelled 

 chromatic substance, which finally disappears along witli all traces of 

 cell boundaries. Nevertheless the former position of tlie nucleus can 

 frequently be distinguished, through the different refractive properties 

 of this region. The barbule thus becomes a horny, almost homogeneous 

 body with no evidence of its original cellular structure, except such as is 

 furnished by the position of the barbicels, the nuclear region, and the 

 presence of pigment patches, to be discussed later. 



Toward the end of tlie process of cornification the feather elements 

 withdraw or shrink away from the non-differentiated cells, which them- 

 selves become more or less shrivelled and cornified (Fig. 21, Plate 5). 

 After the completion of cornification, the feather begins to break forth 

 from the distal end of the feather sheath, a process that begins and con- 

 tinues some time before the formation of the calamus takes place. The 

 barbules, on escaping from the confining sheath, swing about by their 

 own elasticity from the position shown in Plate 1, Figure 6, to tliat 

 seen in Figure 3. 



The process by which the pulp atrophies, having been well described 

 by Davics, will not be discussed here. In the completed feather, as is 

 "well known, all that remains of the dermal pulp is tlie series of dry 

 horny caps found in the quill and a small functional papilla, whicU pro- 

 jects slightly up into the quill through the inferior umbilicus. At the 

 time of molt, this papilla is destined to become active again in the 

 formation of a new feather. 



The cornification of the feather elements has been described by Wald- 

 eyer ('82) and Lwoff ('84). 



IV. The Production of Color in the Feather. 



The researches of Altuin ('54, '54"), Bogdanow ('58), P>rucke ('61), 

 Gadow ('82), Krukenberg ('84), and Haecker ('90) have shown tliat the 

 colors of birds may in general be divided into two classes, (1) those due 

 simply to the presence of a pigment, and (2) the so-called structural 

 colors. Under simple pigment colors they have placed rod, yellow, 

 orange, black, and brown; whereas white, gray, blue, the so-called metal- 



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