=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. =The muscles.=--TECHNICAL NOTE.--Take a piece of intestine from a freshly killed toad, wash it thoroughly and place it in a concentrated solution of salicylic acid in 70% alcohol for 24 hours, then gradually heat until about the boiling-point, when the muscles will fall to pieces. Transfer the preparation to a watch-crystal and tease small bits of isolated muscle with dissecting-needles. Place some of the teased muscle-fibres on a slide, cover with cover-glass, and add a drop of the methyl-green acetic acid. Note the small spindle-shaped _muscle-fibres_. Each one of these fibres is a _cell_ possessing all of the structures common to cells, namely, cell-wall, nucleus, etc. Make a drawing of a few isolated fibres of muscle. From this study of some of the tissues in a toad it will be noted that in the first case we had in the blood separate cells which moved about freely in the plasma. In the second case, that of the epidermis, the cells are fixed edge to edge, thus forming a thin tissue; while in the third and fourth cases, that of the liver and muscle, the cells are not only placed edge to edge, but aggregated into vast masses or bundles, in one case to form the liver and in the other case a muscle. The entire body of the toad is built up of a colony of simple units (cells) combined in various forms to make all the various tissues and organs. CHAPTER IX THE MANY-CELLED ANIMAL BODY.--DIFFERENTIATION OF THE CELL