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.
The skin.—Technical Note.—Keep a live toad or frog in water for some time and note if its skin becomes loose or begins to slip away. If the outer skin, epidermis, comes off, take some of the shed skin and wash it in water, then stain for three or four minutes in a solution of methyl-green and acetic acid (see p. [451]). Cut the pieces of stained skin into small bits and examine one of these under the microscope.
With the low power of the microscope you will note that the skin is made up of a great many flat cells placed edge to edge. Each one has its cell-wall and a central darkly stained nucleus.
Make a drawing of a portion of the toad's skin.
The liver.—Technical Note.—Cut through the fresh liver of a toad, and with a knife-blade scrape from the cut surface some of the liver-cells and place them on a slide with cover-glass.
Examine under the microscope and observe many polygonal cells. Place some of the methyl-green acetic stain under the cover-glass and note, after the cells are stained, that they have definite boundaries and a central nucleus.
Draw some of these scattered liver-cells.