TEGUMENTARY ORGANS 4 IS' 



whose edges the clefts are commencing {fig. 315. f). It would appear,, 

 therefore, that the rootsheaths grow like the shaft of the hair itself 

 not by addition to their surface, but by growth of their deep-seated 

 inner ends. 



Such is the composition of the growing hair ; but the completely 

 formed hair (see § 2. Morphology) presents very great differences in 

 the minute structure of its inner termination. In the first place, the 

 shaft runs out into an irregularly conical mass, like a worn-out 

 painter's brush. It consists, at its extremity, entirely of cortical 

 substance, and the cornification runs in irregular lines into the in- 

 different tissue, which occupies the bottom of the hair sac and 

 represents both pulp and outer rootsheath. The inner rootsheaths 

 terminate above this point, in an irregularly horny layer, which unites 

 with, and is in a manner reflected into, the cuticle of the shaft, which 

 ceases above its brush-like expansion. Finally, the outer rootsheath. 

 in the immediate neighbourhood of the inner, is metamorphosed into- 

 large horny cells, like those of the cellular ecderon. The development 

 of these from the indifferent tissue of the outer rootsheath, may be 

 very clearly traced. The periplast first becomes enlarged and marked 

 off into definite granular areae around each endoplast, and the limits, 

 of each area are metamorphosed into clear horny walls. The cavity 

 which these inclose enlarges, and the endoplast, with its surrounding 

 granular matter, remains attached to one wall, and then eventually 

 disappears, while the cavities enlarge, and their walls thicken into 

 clear horny " cells," which may eventually be detached from one 

 another. 



The whole process of the completion of the root of a hair, then,, 

 is simply a return of the diverticulum of the ecderon, — the meta- 

 morphosis of whose elements, so long as the hair was in course of 

 formation, was guided and determined into distinct forms along 

 certain fixed lines, — to its general tendency to undergo the ordinary 

 cellular metamorphosis over its whole surface. With this return to- 

 its primitive tendencies, the increase of the hair of course ceases, and 

 sooner or later it is pushed out and falls away. 



The spines of the Porcupine, of the Hedgehog, and of the Echidna ^ 

 present in their histological, as in their morphological relations, an 

 interesting approximation to feathers. Externally, they are coated 

 by a cuticle, while the principal mass of their walls consists, at the 

 ends, of a fibrous horny substance ; in the middle, there is added to 

 this a medullary substance composed of polyhedral horny cells. 



The section of the shaft of a fully-formed feather presents exactly 



1 See Brocker (Reichert, Bericht. Miill. Archiv. 1849). 



