STRONG: DEVELOPMENT OF COLOR IN DEFINITIVE FEATHER. 161 
6. Corntfication and Withdrawal of the Feather. 
With cornification, the barb cortex differentiates from the surround- 
ing tissue and the outlines 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 with all traces of 
cell boundaries. Nevertheless the former position of the 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 the 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. 24, 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 that 
seen in Figure 3. 
The process by which the pulp atrophies, having been well described 
by Davies, will not be discussed here. In the completed feather, as is 
well known, all that remains of the dermal pulp is the series of dry 
horny caps found in the quill and a small functional papilla, which 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. Tse Propuction or CoLoR IN THE FEATHER. 
The researches of Altum (’54, 754"), Bogdanow (’58), Brticke (’61), 
Gadow (82), Krukenberg (’84), and Haecker (90) have shown that 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 red, yellow, 
orange, black, and brown; whereas white, gray, blue, the so-called metal- 
VOL. XL. —NO. 3 2 
