In its main features, the process of deve lopment of feathers is identical with that of hairs. A solid diverticulum of the ecderon is first formed, within which the primary change consists in the metamorphosis of cer tain median cells into a cone composed of horny plates. There is thus formed, as in the hair, an outer rootsheath, resembling and con tinuous with the rete vzueosunz, an inner root sheath, and a central papilla, the so-called matrix of the feather (fig. 306.).
The horny rootsheath ( fig. 306. c) attains a very considerable thickness, and instead of stopping short of the mouth of the sac, as in the hair, its outer end is for a considerable time pushed forwards by its basal growth pari passu with that of the feather; so that it eventually projects for a considerable dis tance bey ond the surface. Finally, it opens and allows of the passage of the feather, which grows through it, the horny layer ultimately forming a true rootsheath around the quill. Like the rootsheath of the hair, this structure consists of two layers, an outer (c), denser and harder, and an inner (d), softer and more flexible. The latter from being marked by the projec ting barbs of the young feather has been called the striated sheath. Both layers, however, have the same essential structure, being composed of rounded or polygonal horny plates, whose endoplasts are often distinctly retained even in the outer layers. The histological meta morphosis of the feather will be described below, but the tnanner in which it acquires its ultimate complex general figure requires particular attention. Referring for further de tails to the article AVES, I may state here, that every feather consists of the following parts :— the quit/continuous with the shaft, or central axis of the feather, which supports the hori zontally expanded vane, consisting of numerous long, narrow, flattened laminm ; the barbs or primary rays, pointed at their extremities and arranged with their edges upwards and down wards more or less perpendicularly on the shaft. Arranged in a similar manner on the barbs, are the barbules, which therefore are disposed more or less parallel to the shaft ; from the sides of these, lastly, project short, toothed, curved, interlocking processes.
All parts of the feather are solid, except the quill, which is hollow and occupied only by a dry shrivelled mass, the pith, in its upper part, while below, during life, it receives the pulp. Superiorly, on the under side, %%here the quill joins the shaft, there is a small aperture, which comnmnicates with the interior, with a short canal in the shaft, and with a groove which runs along its under surface.
It may be well to remember that the apex of a barbule resembles in structure one of its own processes ; that of a barb, one of its bar bules ; that of the shaft, one of its barbs.
The development of this complicated or,gan from its matrix or pulp takes place very simply, by a sort of exaggeration of the com bination of hair development with that of the nails, which has already been described as occurring in the spines of the porcupine.
On the surface of the leather-pulp a series of ridges are developed, running pretty nearly parallel with one another from an an tero-posterior groove upon the upper surface, which marks the position of the future shaft, to a line parallel with that groove upon the under surface or the process, which is called the raphe. These ridges, therefore, bound as
many grooves which branch off from the medio-dorsal groove, becoming gradually shal lower, to the raphe. These secondary grooves, as they might be termed, how ever, are not themselves simple ; their walls, the ridges, being again produced into short parallel laminm, and therefore giving rise to tertiary grooves, branching off from the secolidary ones. Now, the whole surface of the matrix being covered by an ecderonic layer in process of conversion into the cor tical and medullary substances of the fea ther, the primary groove becomes filled by the end of the shaft ; the secondary grooves by the terminal barbs, the tertiary' grooves by their barbules, while the processes appear to be outgrowths from these. Were all this conical horny cap to remain entire, the result would be a very complex sort of porcupine's quill ; instead of this, however, it breaks up along the line of each ridge, and so we have a feather.
The extremity of the feather being thus con stituted, how is its remaining length developed ? According to Reichert, the whole pulp elon gates, and as fast as a portion of the feather is completed, the corresponding segment of the pulp dries up, constituting for the vane what has been called the inner striated membrane (e'). However, I believe that this is not the case, the inner striated membrane being, like the outer, a mass of cornified cells detached from the surface of the pulp, just as we shall see the pith of the shaft to be, though this has been also declared by Reichert to be dried up pulp. I believe that the growth of the feather, on the other hand, resembles that of the hairs and nails; viz. the extremity as it is finished, is pushed up by the growth of the base, the pulp only supplying materials from its surface; and I account for the inner stri ated membrane by' supposing that a compara tively imperfect development of horny cell membranes takes place from that surface of the pulp which would otherwise be left bare, when the terminal cone or plume of the feather is pushed away. When the development of the shaft has gone on in this manner for a longer or shorter time, according to the length of the feather, a change takes place. The pri mary groove, which has gradually widened with the width of the shaft (to the exclusion of the secondary grooves, which gradually shorten and ultimately disappear) becoming shallower, extends all round the pulp, and the formation of medullary feather substance ceases, that of cortical substance alone remaining. Thus is the hollow quill formed, and its edges, not quite closing above, leave the minute um bilical aperture by which the inner striated membrane is continued into the " pith "of the quill. This pith is produced by the throwing off of successive transverse horny partitions from the apex of the pulp, as the quill is pushed beyond it; thus protecting itself from the air admitted by the umbilical aperture, and which is visible, occupying the chambers thus formed (fig. 316. G).