Warm-blooded Vertebrates, which breathe by lungs, are covered with feathers, have no teeth but a horny beak, and lay hard-shelled eggs, which are hatched by the warmth of their body. They are adapted for movement in the air, though not all to the same degree. The fore limbs are modified into wings, in which, however, the parts found in Mammals can be recognized. We distinguish in the first place a small thumb, and then in most cases a two-jointed forefinger and a small second finger. _Secondary quills_ (Fig. 28, BB) are attached to the ulna, and _primary quills_ (A, 1–10) to the two metacarpals and the finger joints, while the thumb bears the bastard wing (C). The tail feathers, or _rectrices_, are attached to the last joint of the tail. The body is clothed with stiff, tolerably long _contour feathers_, which conceal the soft short _down_ from view. The bones are hollow and filled with air. Their cavities are connected with _air-sacs_, which are found in all parts of the body, and fill themselves with air from the lungs when the bird begins to fly. In this way its specific gravity is reduced. The body firm, especially the hinder part of it, which is almost immovable; the neck, which may consist of many vertebræ (even as many as twenty-two), can, on the contrary, be turned in many directions. Birds walk entirely on their toes; the metatarsals are fused with one of the rows of tarsals into a “_tarsus_” bone. Tarsus and toes are covered with horny scales. A bird’s egg (Fig. 29) consists of a germinal disc (_h_) from which the young bird develops, and which rests on the yolk, made up of substances serving for the nutriment of the developing bird: the yellow (_a_) and the white (_b_) yolk, as well as the albumen (_c_, _c_^1, “white of egg”) in which lie two twisted cords (_chalazæ_),—finally of protective structures: the shell-membrane (_e_) and the calcareous shell (_f_); _g_ is the air-chamber. [Illustration: FIG. 28.—Wing of the Buzzard. ] [Illustration: FIG. 29.—A Bird’s Egg. ]