836 THE chick. 



Malpighian layer of shorfc cylindrical or cubical cells. At first 

 fche epidermal cap is of uniform thickness all over the papilla; 

 but, as the papilla lengthens, the inner surface of the epithelium 

 thickens along certain lines, so as to form ridges projecting into 

 the papilla. The thickening is due to the formation of a third 

 or intermediary layer of cells, spherical or polygonal in shape, 

 and lying between the epitrichial and Malpighian layers ; and 

 it is from the ridges formed in this way that the feather is 

 developed. 



The thickest and best marked of the ridges runs longitudi- 

 nally, along the upper or anterior surface of the papilla, from base 

 to apex ; while the other ridges, which develop in order from 

 the apex towards the base of the papilla, arise from the sides of 

 this main ridge, and run very obliquely round the papilla to its 

 lower or posterior surface, the ridges of the two sides not quite 

 meeting on the lower surface of the papilla. 



In each ridge, which is thus a solid rod of epithelial cells, 

 the outer cells become elongated and cornified, while the central 

 or axial cells remain comparatively soft. The vascular connec- 

 tive tissue, forming the core of the papilla, now shrinks away 

 from its apex. The outer, or epitrichial, layer of the epidermis, 

 which merely forms a sheath inclosing the papilla, is cast off; 

 and the epithelial ridges or rods, which now alone remain, spread 

 out to form the feather ; the main longitudinal ridge becoming 

 the shaft, and the diverging lateral ridges the barbs ; while 

 minor or tertiary ridges, which arise from the barbs, give rise to 

 the barbules. 



Towards the base of the papilla the epithelial ridges die out, 

 and the entire epithelial investment of the papilla, including 

 both epitrichial and Malpighian layers, becomes converted, by 

 cornification of its cells, into the quill of the feather, which 

 remains open at its lower end for the admission of blood-vessels. 



At the lower end of the shaft, immediately above the quill, 

 the main epithelial ridge widens, and the two sides bend inwards 

 towards each other, and ultimately meet to form a tube, which is 

 continuous below with the quill, and the upper aperture of which 

 persists as the superior umbilicus of the feather. 



The down feathers do not involve the entire length of the 

 papillae, but only their distal or apical portions. The basal 

 portions of the papillae sink deeply below the skin ; a second set 



