STRTJCTUKE OF APPEISTOAGES 317 



present a convex dorsal surface and are concave ventrally. The 

 dorsal surface consists of a horny plate, which is set in a nail matrix. 

 The ventral portion merges with the sides of the upper half, and, 

 as the lower portion is the softer and wears faster, the nail has a sharp 

 point and edge. The matrix is formed by a growth of the Malpigh- 

 ian layer of the cutis. A fold of skin lies over its posterior part. 

 The epidermic cells of the dorsal, or nail, part, and the base of the 

 ventral part grow fast. The outer ceU layer gradually becomes 

 horn-like. 



The spurs are conical with a flat base. The basal part rests upon 

 an enlargement of the shank bone. Soft structure is found between 

 the horny spur and the bone. The upper cells, like those of the nails 

 and of the skin, are constantly being worn or cast off, and new cells 

 push up from the lower layers of cells. These newly formed 

 flattened cells soon become comified. The oldest formation is 

 found at the tip and the yoimgest at the base. 



