THE p;PIDERMIS AND ITS DERIVATIVES. 397 



subjacent dermis. The hair itself arises from a cornification of 

 the cells of the axis of one of the above processes ; and is 

 invested by a sheath similarly formed from the more superficial 

 epidermic cells. A small papilla of the dermis grows into the 

 inner end of the epidermic process when the hair is first formed. 

 The first trace of the hair appears close to this papilla, but soon 

 increases in length, and when the end of the hair projects from 

 the surface, the original solid process of the epidermis becomes 

 converted into an open pit, the lumen of which is filled by the 

 root of the hair. Hairs differ in their mode of formation from 

 scales in a manner analogous to that in which the teeth differ 

 from ordinary placoid scales ; i.e. they are formed in inwardly 

 directed projections of the epidermis instead of upon free 

 papillae at the surface. 



Nails (Kolliker, No. 298) are developed on special regions of the epider- 

 mis, known as the primitive nail beds. They are formed by the cornification 

 of a layer of cells which makes its appearance between the horny and 

 mucous layers of the epidermis. The distal border of the nail soon becomes 

 free, and the further growth is effected by additions to the under side and 

 attached extremity of the nail. 



Although the nail at first arises in the interior of the epidermis, yet its 

 position on the outer side of the mucous layer clearly indicates with which 

 group of epidermic structures it should be classified. 



Dermal skeletal structures. We have seen that in the 

 Chordata skeletal structures, which were primitively formed of 

 both an epidermic and dermic element, may lose the former 

 element and be entirely developed in the dermis. Amongst the 

 Invertebrata there are certain dermal skeletal structures which 

 are evolved wholly independently of the epidermis. The most 

 important of these structures are the skeletal plates of the 

 Echinodermata. 



Glands. The secretory part of the various glandular struc- 

 tures belonging to the skin is invariably formed from the 

 epidermis. In Mammalia it appears that these glands are 

 always formed as solid ingrowths of the mucous layer (Kolliker, 

 No. 298). The ends of these ingrowths dilate to form the true 

 glandular part of the organs, while the stalks connecting the 

 glandular portions with the surface form the ducts. In the case 

 of the sweat-glands the lumen of the duct becomes first 

 established. Its formation is inaugurated by the appearance of 



