2-2 COMPARATIVE ANATOMY. 



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

 however, at first bound together by the enclosing stratum cor- 

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

 stage of development, and they thus appear as if covered with 

 pencil-like hairs. 



By the shedding of the surrounding horny layer the rays or 

 barbs become free (Fig. 12, D, HSt), and if they are all similar to 

 one another, an embryonic down-feather (pluma) is formed. 

 The whole feather-germ, however, does not become divided up into 

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

 retains 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, sec), may retain their character as such throughout 

 life 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, I), 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 comes 

 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 (scapus), to which the barbs are attached on 

 each 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, F, R, HSt, sec). 

 The secondary rays or barbules are so arranged on each barb (IlSt] 

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



In many Birds, each quill of the ordinary feathers of the body bears two 

 vexilla, the second being spoken of as the af tershaft (hyporachis). 



In this manner the contour feathers (penna3) are formed, 

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

 portions of the vexillum usually become very closely united 

 together, so that an extremely strong and resistant though pliant 

 structure is formed. 



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

 in all Birds, and corresponds to the similar process of the casting of 

 the outer skin in Reptiles ; in Mammals there is a continual scaling 

 off of the epidermic cells of a similar nature. 



The feather-covering of Birds must have been acquired in very early 

 geological periods, for Archaoopteryx, found in the Jurassic strata of 

 Havana, possessed well-formed feathers with a very delicate shaft and vane. 



