ORIGIN AND STRUCTURE OF FEATHERS. For the probable origin of plumage as a characteristic and essential element in the class of birds, and its influence on their evolutionary ment, see BIRD. In the individual birds, as now known, the first sign of feathers appears in the embryo, about the fifth day of incubation of the egg. as slight, leaning, conical pimples which arise from the mesoderm. (See BRYOLOGY.) Such a ple gradually sinks into the skin, forming a licle with the papilla rising in its centre: and the walls of this follicle and surface of the la are formed of pighian cells. This tral papilla forms the `feather - pulp,' and its upper portion becomes changed and filler] with blood, forming the tritive organ of the feather. In the space tween the pulp and the walk of the follicle the feather is molded, by the hardening and splitting off of the three superficial layers of cells. The innermost and thinnest tum forms a transparent sheath for the pulp, and persists ultimately as the series of thin caps observable in the stem of a feather. "The middle stratum is the thickest, and becomes the feather itself, while the outermost forms a transparent and coherent cylindrical sheath, which incloses the growing feather, giving it its well-known spine-like appearance ['pin-featherl. until, peal ing off as scurf, it sets free the rani (vanes) of the young product" (Newton). This is the his tory of the first growth, but the process is sub stantially the same for all feathers, which arise from the same pulps. For an account of the periodical shedding and renewal Of plumage, see .:\loi.rixe. See also INTEGUMENT; SI:ELETON.
A complete feather consists of a shaft and a vane. The shaft is made up of the cylindrical hollow barrel or calamus which extends to the beginning of the vane, \•here it is succeeded by the opaque, pith-filled squarish stem or raellis: in many birds the feather also bears upon the inside of its calamus an aftersha ft, or hypo rachis, which is a counterpart of the main feather, and occasionally, as in the omen, may equal it. The vane or web is the blade-like ex pansion along the sides of the distal part of the feather, and consists of several elements: first, a row of horny lamelli, called barbs or mini, which are wedge-shape in section, the thin edge being turned toward the bird's body. Their number varies: a crane's wing-feather, 14 inches long, has about 650 in its inner web. These barbs bear
on each side similar lamelli. called barbules or radii, very minute, and exceeding a million in number for such a feather as. the crane's; and each one of these has its upper margin turned over, like a flange, toward the rachis; further more, the end of each radius on that side of the minus which looks toward the tip of the feather is split up into a fringe of hooks that reach over the radii of the next forward row. and hook on to their flanges. thus connecting them all into the firm, springy, and almost air-tight fabric pre sented by most surface or 'contour' feathers —especially a 'flight-feather,' as a wing-quill (remex) or a tail-quill (rectrix). Other kinds of feathers, as the soft and fluffy underlying downs. the hair-like, degenerate filoplumes, and scaly or wiry feathers, exhibit the absence of some or all of these connecting parts, or their modifica tion.
The purpose of the feathers is mainly protec tion from cold and wet; they arc exceedingly warm because their substance ( resembling horn) is a poor conductor of beat. checking radiation, and because of the air which they contain or entangle forming a blanket of dry air about the body. 'Co enable them the better to resist wet, most birds are provided with a store of grease in the oil gland (see BIRD), with which they often anoint the plumage. Moreover, the skin of many birds, espe cially aquatic species of cold climates, is covered with a thick coating of clown feathers, each of which is composed of a very small soft tube lying in the skin, from the interior of which arises a minute tuft of soft filaments, without any central shaft. This downy covering secures warmth without weight, like the soft fur at the base of the hair of Arctic mammals, and is an adaptive sur vival of the earliest form of plumage.
The embryo within the shell. and afterwards in the nest, is clothed with 'nestling down,' which consist::, of short. incomplete, nearly colorless feath ers, called neossoptiles, which in some birds grow only on limited spots, or, rarely, do not appear at all; and sometimes they resemble hairs or bristles more than feathers. This first coat soon disap pears. being pushed off in the first molt by the growth of the real plumage, which arises from the same places and areas as did the nestling down.