xm PHYLUM CHORDATA 355 



furthest from the body of the Bird spring two sets of barbules, a 

 proximal set (C) directed towards the base of the feather, and a 

 distal set (D) towards its tip. Owing to their oblique disposition 

 the distal barbules of a given barb cross the proximal barbules of 

 the next, each distal barbule being in contact with several proximal 

 barbules of the barb immediately distal to it (A). The lower edge 

 of the distal barbule is produced into minute booklets (D): in 

 the entire feather the booklets of each distal barbule hook over 

 prominent flanges of the proximal barbules with which it is in 

 contact (A, B). In this way the parts of the feather are so 

 bound together that the entire structure offers great resistance 

 to the air. 



Among the contour feathers which form the main covering of the 

 Bird and have the structure just described, are found filoplumes 

 (Fig. 964, B.), delicate, hair-like feathers having a long axis and a 

 few barbs, devoid of locking apparatus, at the distal end. Nestling 

 Pigeons are covered with a temporary investment of down-feathers 

 (C), in which also there is no interlocking of the barbs : when 

 these first appear each is covered by a horny sheath like a glove- 

 finger. 



Feathers, like scales, arise in the embryo from papillae of the 

 skin (Fig/966, A, Pap.), formed of derm with an epidermal covering. 

 The papilla becomes sunk in a sac, the feather-follicle (B, F), from 

 which it subsequently protrudes as an elongated feather-germ 

 (F K\ its vascular dermal interior being the feather-pulp (P). 

 The Malpighian layer of the distal part of the feather-germ pro- 

 liferates in such a way as to form a number of vertical radiating 

 ridges (C, Fed SM") : its proximal part becomes uniformly thickened, 

 and in this way is produced the rudiment of a down-feather, having 

 a number of barbs springing, at the same level, from the distal end 

 of the quill. The horny layer of the epidermis (H S (<;')) forms 

 the temporary sheath wnich is thrown off as the feather grows and 

 expands. The pulp of the permanent feather (D, F') is formed 

 from the lower or deep end of that of the down -feather, and its 

 development is at first similar, but, instead of the ridges of the 

 Malpighian layer remaining all of one size, two adjacent ones out- 

 grow the rest and become .the rachis ; as the latter elongates it 

 carries up with it the remaining ridges, which become the barbs. 



The feathers do not spring uniformly from the whole surface of 

 the body, but from certain defined areas (Fig. 967), the feather 

 tracts or pterylaz (sp. pt., Mi.pt., &c.), separated from one another by 

 featherless spaces or apteria (v. apt., &c.), from which only a few 

 filoplumes grow. The feathers are, however, long enough to cover 

 the apteria by their overlap, and the body is thus completely 

 covered with a thick, very light, and non-conducting investment. 



In the wings and tail certain special arrangements of the feathers 

 are to be distinguished. When the wing is stretched out at right 



A A 2 



