=The Ameba.=--Of all the animal kingdom, the _minute creatures that can be seen only with a microscope_ are most different from man. One of the most interesting of these is the _a-me′ba_ (Fig. 5; spelled also _amœba_, see Animal Biology, Chap. II). _A thousand of them placed in a row would hardly reach an inch._ Some may doubt whether the ameba is a complete animal. Study the figures of it, and no head, or arms, or legs, or mouth can be found. It appears, when still, to be merely a _lump of jelly_. But the ameba can _push out any part of its body as a foot_, and move slowly by rolling its body into the foot. _It can put out any part of its body as an arm_, and take in a speck of food; or, if the food happens to be near, the ameba can _make a mouth in any part of its body_, and swallow the food by closing around it (Animal Biology, Fig. 12). The ameba has no lungs, but _breathes with all the surface of its body_. Any part of its body can do anything that another part can do. When the ameba grows to a certain size, it multiplies by squeezing together near the middle (Animal Biology, Fig. 13) _and dividing into two parts_. Amebas have not been observed to die of old age; starvation and accident aside, they are immortal. [Illustration: FIG. 6.--A WHITE BLOOD CELL, magnified; forms noticed at intervals of one minute.] =The Ameba and Man Compared.=--The microscope shows us that the skin, the muscles, the blood,--in fact, _all parts of the body_,--_contain numberless small parts called cells_. These cells are continually changing with the activities of the body. One of the most interesting kinds of cells we shall find to be the _white blood cells_, or corpuscles. One is shown in Fig. 6, with the changes that it had undergone at intervals of one minute. The thought readily occurs that _these cells, although part of man’s body, resemble the ameba_ that lives an independent life. A man or a horse or a fish--in fact any animal not a protozoan--has something of the nature of a colony, or collection, of one-celled animals. We are now prepared to understand a little as to how the body grows, and how a cut in the skin is repaired. _The cells take the nourishment brought by the blood, use it, and grow and multiply like the ameba._ Thus new tissue is formed. All animals and vegetables--that is to say, all living things--are made of cells. [Illustration: FIG. 7.--DIAGRAM OF A CELL. _p_, protoplasm; _n_, nucleus; _n′_, nucleolus.]