376 TEGUMENTARY ORGANS 



&c., until at last the cavity is circular as at the apex. The boundary 

 of the quill cavity is immediately formed by medullary substance ; 

 but the cortical substance follows to a certain extent the contour of 

 the inner cavity, so that in a transverse section of the middle of the 

 quill the cortical substance presents the same general outline as the 

 medullary, though its processes and insections are less marked. 



In the adult condition, the central cavity is filled by an irregular 

 horny mass, which Reichert and Brdcker regard as the dried-up pulp, 

 but which is probably, as in the feather {vide infra), simply the last 

 horny product of the pulp, filling up the space which the latter once 

 occupied ; for it is certain that every portion of the porcupine quill 

 has, like every portion of a feather, at one time constituted a cap over 

 the corresponding portion of its pulp. The pulp, in fact, commences 

 like that of a feather, as a smooth conical process upon which the 

 ape.x of the quill is moulded. As it grows, however, the pulp assumes 

 an angular form, and then, as that of a feather would do, becomes 

 produced into lamellae. By the constant production of new elements 

 at the surface of these lamellae and their cornification, the " quill " is 

 produced, and retains internally the impression of the mould on 

 which it was formed. Apart from the arrangement of the lamella;, 

 the principal difference from a feather which the '' quill " presents, is 

 simply that it does not, as it is formed, split up along the lines of the 

 lamellce of the pulp. 



In its main features, the process of development oi feathers is 

 identical with that of hairs. A solid diverticulum of the ecderon is 

 first formed, within which the primary change consists in the meta- 

 morphosis of certain median cells into a cone composed of horny 

 plates. There is thus formed, as in the hair, an outer rootsheath, 

 resembling and continuous with the rete vincosum, 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 distance beyond 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 (f), denser and harder, and an inner 

 {d), softer and more flexible. The latter from being marked by the 

 projecting barbs of the young feather has been called the striated 



