Sauropsida : Class Aves 509 



base of the follicle loosens, and the feather comes out. A new feather 

 is developed in the old follicle. 



The development of a feather is initiated by formation of a papilla 

 involving both dermis and epidermis (Fig. 399A). The papilla elon- 

 gates and at the same time sinks bodily into the dermis, carrying the 

 surrounding epidermis along with it, and thus forming the follicle 

 (Fig. 399B, C). The dermal core of the papilla is richly vascular, its 

 blood providing nourishment for the developing feather. Then the 

 epidermis of the protruding region of the papilla becomes arranged 

 according to the pattern of the prospective feather. In the case of a 

 down-feather, it splits into numerous longitudinal divisions, each of 

 which becomes shaped into a slender rod (Fig. 400). The outermost 

 layer of epidermal cells remains undifferentiated, forming a sheath 

 (periderm) enclosing the developing feather. The epidermal rods 

 eventually cornify (i.e., become transformed into horny substance) 

 and, rupturing the sheath, emerge and separate to become the tuft of 

 barbs of the down-feather. The dermal core of this portion of the young 

 feather dies, dries, and is cast off with the ruptured sheath. The epi- 

 dermal wall of the proximal portion of the feather cornifies to become 

 the quill, whose dermal core is gradually reduced to a dead, dry, spongy 

 pith. 



A follicle of a nestling down-feather may later give rise to one of 

 the larger contour feathers. The follicle deepens and enlarges and a 

 new and larger papilla consisting, like the first, of epidermis with a 

 dermal core grows out from its bottom. The development of the con- 

 tour feather is necessarily a more complex process than that of a down- 

 feather, but it follows the same principle. The epidermal tissue of the 

 papilla becomes arranged so as to map out the pattern of the feather 

 (Fig. 399D). Rachis, barbs, and barbules take shape in the living 

 tissue but are confined within a delicate epidermal sheath. The differ- 

 entiation begins at the apex of the papilla and extends toward the 

 base. At a late stage, it is as if a finished feather had been carefully 

 rolled up and wrapped in cellophane. Even the development of the 

 minute hooks and the interlocking of barbules is all perfected while 

 the vane is tightly rolled up within the sheath. The tip of the feather 

 matures first, ruptures the sheath, emerges, and unrolls. The remainder 

 of the feather is gradually pushed out as the completion of differentia- 

 tion progresses proximally. At any given level of the feather, cornifica- 

 tion does not begin until the structures in that region are well formed, 

 and is not completed until they have attained definitive size, propor- 

 tions, and pigmentation. Completely cornified tissue is no longer alive. 



The essential feathers concerned in flight are long and stiff, and 

 the broad, smooth vane is so compactly built that air does not leak 



