STRUCTURE AND CLASSIFICATION OF BIRDS 



Feathers ' 



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 



' H. R. Davies, ' Beitrag zur Entwioklungsgesehichte der Feder,' Morph. J.B. 

 xiv. 1888, p. 368, and 'Die Entwioldungsgesoh. d. Feder,' &c. ibid. xv. 1889, 

 p. 560 ; C. E. Hennicke, ' Die Entwicklung d. Feder,' Monatsschr. deutsch. Ver. 

 Vogelsch. xiv. 1889, p. 223 ; K. Klee, , ' Bau und Enlwioklung der Feder,' 

 Zeitschr. f. d. ges. Naturw. lix. p. 110 ; see also Gadow, article ' Feather' in 

 Newton's Diet, of Birds. 



