6 STRUCTURE AND CLASSIFICATION OF BIRDS 



Feathers l 



A. bird may be known by its feathers ; to define a bird it 

 is only necessary to refer to its covering of feathers. No 

 other animal has any structures comparable to a well- 

 developed feather. It is true that the filo-plumes are really 

 little more than hairs. But the processes of development 

 serve to place a fundamental barrier between the two kinds 

 of structures. 



A hair commences as a thickening of the stratum 

 Malpighii, which grows downwards into the dermis ; a feather 

 is from the first a slight papilla involving the outer layers of 

 the epidermis as well as the stratum Malpighii, a papilla 

 which is surrounded by a circular depression. This papilla 

 gradually sinks down into the skin and assumes a cylindrical 

 form. The cells of the Malpighian layer commence to pro- 

 liferate vigorously, and form a series of thickened folds 

 disposed radially to the longitudinal axis of the feather 

 papilla, and towards the central pulpa. These radially 

 arranged masses of cells undergo a process of cornification, 

 free themselves from the overlying cells of the horny layer of 

 the epidermis, and produce a bundle of horny fibres the 

 embryonic down. The feathers may retain this embryonic 

 character throughout life, or further changes may take place. 

 This consists in the formation below the first feather follicle 

 of a second in continuity with it ; in this a feather is 

 developed, which may be a down feather, like the first formed, 

 or may grow into one of the stronger varieties of feathers to 

 be described presently. In either case the growing feather 

 pushes the down before it, and the latter is ultimately thrown 

 off. 



The structure of feathers has been described at length by 



1 H. E. DAVIES, ' Beitrag zur Entwicklungsgeschichte der Feeler,' Morph. J.B. 

 xiv. 1888, p. 368, and 'Die Entwicklungsgesch. d. Feder,' etc. ibid. xv. Issn, 

 p. 5GO ; C. R. HEXXICKE, ' Die Entwieklung d. Feder,' MonatsscJn: dcntscli. Ver. 

 Vogclscli. xiv. ISN'.I, p. 223 ; K. KLKK, ' Ban und Enlwicklung der Feder,' 

 Zeitsclir. f. d. gcs. Naturw. lix. p. 110 ; see also GADOW, article ' Feather' in 

 NEWTON'S Diet, of Birds. 



