In shape, those of the mammalia are circular discs, mostly with a concave centre, though the camel has oval ones; those of the birds are more or less oval and convex; those of the reptiles are decidedly oval, very thin, and have the nucleus projecting; and those of the fishes are oval and mostly convex. During the process of coagulation the blood corpuscles run together into a series of rows, just as if a heap of pence had been piled on each other and then pushed down, so that each penny overlaps its next neighbour. These objects are illustrated by six examples on Plate X. Fig. 2 is human blood, showing one of the white corpuscles; Fig. 3 is the blood of the pigeon; Fig. 4, of the _Proteus anguínus_; Fig. 5, of the tortoise; Fig. 6, of the frog, showing the projecting nucleus; and Fig. 7, of the roach. The blood possesses many curious properties, which cannot be described in these few and simple pages. In the centre of Plate X. is a large circular figure representing the membrane of a frog’s foot as seen through the microscope, and exhibiting the circulation of the blood. The mode of arranging the foot so as to exhibit the object without hurting the frog is simple enough. Take an oblong slip of wood,--my own was made in five minutes out of the top of a cigar-box,--bore a hole about an inch in diameter near one end, and cut a number of little slits all round the edge of the wooden slip. Then get a small linen bag, put the frog into it, and dip him into water to keep him comfortable. When he is wanted, pull one of his hind feet out of the bag, draw the neck tight enough to prevent him from pulling his foot back again, but not sufficiently tight to stop the circulation. Have a tape fastened to the end of the bag, and tie it down to the wooden slide. Then fasten a thread to each of his toes, bring the foot well over the centre of the hole, stretch the toes well apart, and keep them in their places by hitching the threads into the notches on the edge of the wooden strip. Perhaps an easier plan is to secure the threads by drops of sealing-wax when in the desired position. Push a glass slide carefully between the foot and the wood, so as to let the membrane rest upon the glass, and be careful to keep it well wetted. If the frog kick, as he will most likely do, pass a thin tape over the middle of the leg, and tie him gently down to the slide. Bring the glass into focus, and the foot will present the appearance so well depicted in the engraving. The veins and arteries are seen spreading over the whole of the membrane, the larger arteries being often accompanied by a nerve, as seen in the illustration. Through all these channels the blood continually pours with a rather irregular motion, caused most probably by the peculiar position of the reptile. It is a most wonderful sight, of which the observer is never tired, and which seems almost more interesting every time that it is beheld.