The nails originate from the epidermis, of which they are a modification. The nails of the toes are bent downward. They 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 Malpighian 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 cell 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 cornified. The oldest formation is found at the tip and the youngest at the base. EMBRYOLOGY OF THE CHICK That a new individual may be brought into existence, there must be accomplished the union of the male element, or _spermatozoon_, with the female element, the _ovum_. This union is called _fertilization_. In the fowl this fertilization is accomplished at the anterior portion of the oviduct, after the calyx has ruptured and discharged its yolk, and before the albumen has been formed around it. The blastoderm is found on the surface of the yolk. One spermatozoon is all that is required; in fact, only one can be used in this union.