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.
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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.
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Fig. 7.—Diagram of a Cell.
p, protoplasm; n, nucleus; n', nucleolus.
A living cell always contains a still smaller body called a nucleus (Fig. 7). There is sometimes a small dot in the nucleus, called the nucleolus. The main body of the cell consists of the living substance called protoplasm, containing nitrogen. Usually, but not always, there is a wall
