animals and 1/2500 inch in reptiles. Sometimes, as we are examining the white blood corpuscles under the microscope, we shall notice that they behave in a similar manner to the Proteus Animalcule, which we described in our chapter on Pond Life. A kind of creeping motion takes place and the corpuscle loses its circular shape. In our own blood this movement lasts but a very few moments, in frogs’ blood, by keeping our slide moderately warm, we may witness the movement for some time. It is perfectly easy to watch the circulation of blood in the foot of a frog. For this purpose we require a piece of apparatus known as a frog plate, this is merely a flat brass plate perforated with a number of holes, through which tape is passed to bind the frog down and having also a hole the size of the opening in the microscope stage, into which a circle of glass is usually fitted. Such a plate may be made of wood and will serve our purpose quite as well as the more expensive brass plate. We must bind our frog firmly, but not too tightly with wet rag, leaving one leg exposed. This free leg must now be fastened in such a manner that one of the webs between its toes comes over the opening in the plate, finally the toes must be carefully tied with string so that they remain apart and the web is fully expanded. Having fixed our frog in a satisfactory position let us examine his blood-vessels under a fairly high power. We shall find that there are large and small vessels in his webbed foot; we will devote our observation to the small vessels, called capillaries, from the Latin _Capillus_, a hair, because of their small size. Probably the blood will not flow when we make our first examination and this points to one of two things, either the frog is bound too tightly or he has not recovered from the alarm he experienced at his treatment. In the latter event he will not be long recovering and his blood circulation will soon be in full working order; in the former case, we must loosen his bandages. In some of the very small blood-vessels we shall notice that the blood always flows in one direction, in others it does not appear to have any definite direction. In either case, however, we can see the red corpuscles flowing rapidly along the central stream of the blood-vessel whilst the white corpuscles travel much more slowly along the sides.