374 TEGUMENTARV ORGANS 



laminated fibrous horny mass, which is formed and increased by 

 apposition from the subjacent process of the enderon, supported by its 

 bony axis — a process of the frontal bone. The enderon has neithei' 

 \'illi nor lamellje, presenting only small irregular ridges (Gurlt). 



The horn of the rhinoceros is commonly said to be constituted by 

 a mass of hairs which have coalesced. However, it consists of an 

 aggregation of tubes, round which the horny matter is arranged in 

 concentric laminae, as in the horny excrescence of the horse's leg ; and 

 as there is no evidence of its having ever been enclosed within a sac, 

 it is more probable that it belongs to the series of the claws and nails. 



Glands, liairs, and feathers. — The Hairs and Spines of mammals, 

 the Feathers of birds, and the Integumentary Glands agree in one 

 essential point, that their development is preceded by that of an 

 involution of the ecderon, within which they are formed, and by which 

 the former are, at first, entirely enclosed. 



At an early period, the rudiments of the hairs, and those of the 

 cutaneous glands of a fcetal mammal, are indistinguishable. They 

 alike consist of solid processes of the ecderon, consisting of a 

 homogeneous matrix, in which lie closely-set endoplasts, bounded 

 internally by a clear, narrow, transparent " basement membrane,'' 

 which at once separates them from, and connects them with, the 

 enderon.i Externally these processes are continuous with the rete 

 mucosum of the ecderon. In the foetal lamb, in which I have carefully 

 traced the development of these processes, they increase in size 

 without change of structure, until, in the ordinary hairs, they have 

 attained a length of ylo inch ; for the vibrissa, that of -^V inch. 

 Having reached this length, it is seen that an accumulation of the 

 indifferent tissue of the enderon has taken place around their ccecal 

 ends, which gradually become pushed in, so that, from being rounded, 

 they appear truncated in section, and present a bulb with a hemisphe- 

 rical involution, the rudiment of the papilla;. In the ordinary hair 

 no special accumulation of indifferent tissue takes place around the 

 body of the involution ; but in the vibrissae, which are ultimately to 

 possess a thick outer capsule, its foundation appears in this form, and 

 a capillary loop may be seen penetrating the rudimentary papilla. 



In the furthest advanced vibrissa the tissue of the axis of the sac 

 was converted into horny cells, the rudiment of the " fenestrated " or 

 of the inner, horny rootsheath. Over the papilla the rudiment of the 

 hair shaft was indicated by a conical process, horny at its apex and 

 marked by radiating lines. Finally, on each side of the neck of the 



1 The further development of the glands will be most conveniently considered, together 

 with their histological structiu-e, below. 



