2-2 ('OMI'AKATIVK ANATOMY. 



central pulp-substance. give rise to a tuft of horny rays, which aiv, 

 however, at lii-st bound together by the mi-losing stratum cor- 

 neum. Most Birds are hatched when the feathers are in this 

 stage of development, and they thus appear as it' covered with 

 pencil-like hairs. 



By the shedding "f the surrounding horny layer the rays or 

 b.u-bs become free (Fig. 12, 1), //N/'. and if they are all similar to 

 ono another, an embryonic down-feather (phi m a) is formed. 

 Tin- \vhole feather-germ, however, does not become divided up into 

 barbs in this manner: its lower portion, embedded in the skin, 

 r. tains a more uniform character and forms the quill (calamus) 



(FSp). 



The embryonic down-feathers (Fig. 12, E), on the individual 

 barbs of which smaller secondary rays or barbules ' become 

 developed (sec, scr), may retain their character as such throughout 

 lite or may be replaced by definitive feathers. In this case a second 

 follicle early arises from the base of the follicle of the down-feather, 

 with which it is connected by a cellular cord, and which it closely 

 resembles in structure (Fig. 12, D, F 1 }. The papilla developing 

 within the interior of this new follicle grows rapidly, gradually 

 pushes the base of the down-feather out of its follicle, and conns 

 to the surface. Each definitive feather at first closely resembles a 

 down-feather in structure, and consists of a tuft of similar rays or 

 barbs provided with barbules. In the course of further growth. 

 however, one of the rays becomes rapidly thickened, and forms a 

 main axis or stem ('sea pus), to which the barbs are attached on 

 ach side. The proximal or basal portion of the scapus which bears 

 no barbs is called the quill (calamus), and the distal part, to 

 which the barbs are attached, the shaft (rachis). The barbs 

 together constitute the vane (vexillum) (Fig. 12, K. /i', //N/. sec). 

 The secondary rays or barbules are so arranged on each barb (7/S/) 

 as to make the latter resemble an entire feather in appearance. 



In many Birds, each quill el 1 tin- ordinary feathers of the Imdy hears t\\o 



\ .\ill.-i, tin- second lieinx spoken of a- (In- a f t e r.-ha ft (hyporachis). 



In this manner the contour feathers (pennaO are formed, 

 such, for instance, as those on the wings and tail. The individual 

 portions of the \e\illnm usually become verj closely united 

 together, so thai an extremely strong and re-i-tant though pliant 



structure is (brined. 



A periodic casting of feathers, or "moulting." takes place 

 in all Ilinls, and corresponds ( () the similar process of the castingof 

 I he ou tei- skin in Ueptiles ; in Mammals there is a continual scaling 

 off of the epidermic cells of a similar nature. 



The feather-covering !' Ilinls must have limi :n'(|iiiivcl in vrrv rarlv 

 geological pi-ri. ,U, l,.r A r<-li;rn|i| cry x, I'mnnl in Ilic .1 IHVI-HC >ti'ala ..)' 

 l'.a\ari:i. \> ------ JSCcl \\rll-lnnin-cl 1'eallnT- \\illi a \cry delicate shaft and vaiie. 



