384 



ZOOLOGY 



The papilla becomes sunk in a sac, the feMher-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, Fal SM') : its proximal part becomes uniformly thickened, 

 and ill 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 (S S (sc')) forms 

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

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





Flo. 1020.— Pterylosis of Columba livia. A, ventral ; B, dorstil. aX. pt. alar pteryla or wing- 

 tr.act ; c.p^. cephalic pteryla or head-tract ; erf. ^(. caudal pteiyla or t.ail-tract ; cr. pi. crural 

 pteryla; ci-.a-pt, Qcrvicsl apterium or neck-space; fm.pt. femoral pteryla; Am. pf. humeral 

 pteryla ; lat. apt. lateral apterium ; sji. pt. spinal pteryla ; v, aptt. ventral apterium ; v. jii. 

 veutx-al pteryla. (After Nitszch.) 



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 1026), the feather 

 tracts or pteryla', (sp.pt., hu. ft., &c.), separated from one another by 

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

 flloplumes 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. 



