EPIDERMAL STRUCTURES. 



95 



Fig. ioi. Part of a feather, enlarged, a, 

 portion of shaft showing a part of a barb with 

 its barbules ; b, two barbules greatly enlarged. 



shaft with the quill. The vane supported by this after shaft is 

 usually more downy than 

 the others. 



Down-feathers differ 

 from contour-feathers in 

 the absence of a shaft, 

 the barbs arising directly 

 from the end of the quill ; 

 these barbs never inter- 

 lock, but remain soft and 

 ] 'ce from each other. In 

 pin-feathers (filoplumes) 

 there is merely the devel- 

 opment of a hair-like shaft 

 without barbs. 



Except in the pen- 

 guins and some ratite birds, feathers are not uniformly distrib- 

 uted over the whole surface of the body, but occur in well-marked 



feather-tracts or pterylae, the rest of 

 the surface (apteria) being sparsely 

 covered with down- or pin-feathers. 

 The tertiary penguins also possessed 

 feather-tracts, so that their absence in 

 existing penguins must be a secondary 

 character. The arrangement of the 

 feather-tracts is of importance in the 

 classification of birds. 



In development down-feathers pre- 

 cede contour-feathers. There first 

 appears in each spot where a down 

 feather is to develop a rapid multipli- 

 cation of dermal cells, thus producing 

 a rudimentary papilla, over which the 

 epidermis, elsewhere consisting of 

 basal layer and epitrichium, becomes 

 several cells in thickness. By contin- 

 uous growth the papilla becomes long 

 and cylindrical, projecting from the body, the axial derma form- 



^^5*^^^^^^:s5» 



Fig. 102. Feather tracts in 

 young of common crow ( Corvus 

 americanus'). 



