3:W THE EXOSKELETON. 



quill is formed by a cornification in the form of a tube of both layers of 

 the epidermis at the base of the papilla. The quill is open at both ends, 

 and to it is attached the vexillum or plume of the feather. In a typical 

 feather this is formed at the apex of the papilla from ridge-like thicken- 

 ings of the mucous layer of the epidermis, arranged in the form of a 

 longitudinal axis, continuous with the corn! tied mucous layer of the quill, 

 and from lateral ridges These subsequently become converted into the axis 

 and barbs of the' plume. The external epidermic layer becomes converted 

 into a provisional horny sheath for the true feather beneath. 



On the completion of the plume of the feather the external sheath is 

 thrown off, leaving it quite free, and the vascular core belonging to it 

 shrivels up. The papilla in which the feather is formed becomes at a 

 very early period secondarily enveloped in a pit or follicle which gradually 

 deepens as the development of the feather is continued. 



Hairs (Kolliker, No. 298) are formed in solid processes of the 

 mucous layer of the epidermis, which project into the 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 epi- 

 dermis 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 di- 

 rected projections of the epidermis instead of upon free papillae at 

 the surface. 



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

 epidermis, 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 epi- 

 dermic and derrnic 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 structures 

 belonging to the skin is invariably formed from the epidermis. In 

 Mammalia it appears that these glands are always formed as solid in- 



