ii THE SKIN AND ITS DEEIVAT1VES 71 



horny ■ plates arranged edge to edge — as in Chelonians or as on 

 the ventral side of the body in Crocodiles or the dorsal surface of 

 the head in Snakes and Lizards — or, finally, they may overlap like 

 slates on a roof as is the case on the bodies of Lizards and Snakes. 

 Occasionally, as in certain Lizards, individual scales may become 

 greatly thickened and assume a conical spike-like form. 



The individual scale arises in development (Fig. 41) as a slight 

 elevation of the surface beneath which the dermal connective tissue 

 is somewhat concentrated. The epidermis covering the projection 

 develops a well-marked cuticle. As development goes on the 

 epidermis increases much in thickness and the cells of the outer 

 layers become entirely cornified so as to form a horny plate or scale 

 — supported by the underlying tough condensed portion of the dermis. 



It will be borne in mind that such typical reptilian scales have 

 to be sharply distinguished from the morphologically quite different 



Fig. 41. — Early stage in the development of the scale of a snake as seen in a longitudinal 

 section perpendicular to the surface of the skin. 



scales developed in the dermis in fishes. The ordinary reptilian 

 scales serve mainly to protect the body from mechanical violence 

 and from desiccation. 



Feathers. — In the homoiothermic Birds, where the body is kept 

 at a constant temperature usually higher than that of the surround- 

 ing atmosphere, the scales have become for the most part replaced by 

 fluffy feathers which with the air entangled in their interstices form 

 an admirable non-conducting envelope to retard the loss of heat by 

 radiation, or convection, from the surface of the body. 



The rudiment of the feather begins (Fig. 42, A) as a slight 

 thickening of the epidermis resting upon somewhat condensed 

 dermis. The rudiment in fact differs little from that of a normal 

 scale. The rudiment comes to project backwards (B) and then in- 

 creases in length (C), projecting freely tailwards while its now 

 relatively narrow base of attachment becomes sunk below the general 

 surface into a pit or follicle. 



The rudiment now consists of a core of dermis surrounded by 

 thick epidermis. The epidermis becomes incised along its axial 

 surface by deep longitudinal grooves which divide its deeper portions 

 into longitudinally arranged masses (Fig. 42, D, b), the rudimentary 

 barbs, while leaving the superficial portion as a continuous sheath 

 (sh.). The grooves in question do not reach to the base of the rudi- 



