=Protoplasm.=--The protoplasm, which is the essential substance of the typical animal cell and hence of the whole animal body, is a substance of very complex chemical and physical make-up. No chemist has yet been able to determine its exact chemical constitution, and the microscope has so far been unable to reveal certainly its physical characters. The most important thing known about the chemical constitution of protoplasm is that there are always present in it certain complex albuminous substances which are never found in inorganic bodies. And it is certain that it is on the presence of these substances that the power possessed by protoplasm of performing the fundamental life-processes depends. Protoplasm is the primitive physical basis of life, but it is the presence of the complex albuminous substances in it that makes it so. The physical constitution of protoplasm seems to be that of a viscous liquid containing many fine globules of a liquid of different density and numerous larger globules of a liquid of still other density. Some naturalists believe the fine globules to be solid grains, while still others believe that numerous fine threads of dense protoplasm lie coiled and tangled in the clearer, viscous protoplasm. But the little we know of the physical structure of protoplasm throws almost no light on the remarkable properties of this fundamental life-substance. CHAPTER VIII CELLULAR STRUCTURE OF THE TOAD (OR FROG) LABORATORY EXERCISE =The blood.=--TECHNICAL NOTE.--The blood of a frog can be studied as it flows through the small vessels in the membranes between the toes while the animal is alive. Place a frog on a small flat board which has had a hole cut near one end, and with a piece of cloth bind it to the board. Spread the web between two toes over the hole in the board and keep it in place with pins. This done, examine the distended web under the compound microscope first with low then with higher power, and observe the blood-vessels and the blood circulating in them. For a further study of the blood kill a toad or frog and place a drop of the blood on a slide with a cover-glass over it. Put the prepared slide under the microscope and note that the blood, which as seen with the unaided eye appears to be a red fluid, is made up of a great many yellowish elliptical disks or _cells_, the _blood-corpuscles_, floating in a liquid, the _blood-plasma_. Here and there you may notice _amœboid blood-corpuscles_. These are irregular-shaped cells which move about by thrusting out pseudopodia. They look like some of the unicellular animals, as the _Amœba_. Can you distinguish a nucleus and cell-wall in the blood-cells? Make drawings of these blood-cells.