In the downy feathers (Pl. IX. fig. 17) the barbs are not furnished with the pinnæ, but present simply whorls of minute spines (fig. 18). The _bones_ of birds present the same general structure as that of mammals, the lacunæ being, however, more numerous and smaller. The _blood_ of birds (Pl. IX. fig. 2) differs entirely from that of mammals, in the red corpuscles being oval instead of circular, and convex instead of concave; and each contains a distinct oval and granular nucleus. REPTILES.--In reptiles, as the frog, toad, or water-lizard (_Tríton_), the bone-corpuscles or lacunæ are larger and more numerous than in either of the former classes; and the blood-corpuscles (Pl. IX. fig. 3) are comparatively very large, oval, rather concave, and contain a large granular nucleus. The smooth water-newt or triton, properly called _Lissotríton punctátus_, is a very interesting animal in a microscopic point of view. It may be found in most ponds; and if several are removed in a net, and kept in a large glass jar, with water-plants, they will live for a long period. In the spring or early summer they will deposit their eggs upon the aquatic plants, generally on the under surface of a leaf, which they bend downwards, so as to protect them. The eggs or ova, are about half the size of a pea, and consist of a sac containing a transparent liquid, with a yellowish globule within. After a time these eggs will hatch, and the larvæ or young newts must be removed from the water, otherwise the parents will devour them. If one of these larvæ, which resemble little fish in appearance, be placed with a little water in the “live-box,” and the cap be very gently pressed down, so as to fix the body of the animal, the circulation of the blood may be very beautifully seen in either the fringe-like gills, which are placed on each side of the neck, or in the tail, a low power being used; at the same time the beautiful stellate pigment-cells of the skin will be observed. The structure of the rudimentary spinal column, which runs down the middle of the back, and consists of simple large cartilage-cells, may also be made out, when the animal is dead, by a little dissection with the aid of needles.