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348 them back to life. These blood-cells that I have described are called "human amœbæ," or the amœbæ of man. Those that are found in the blood of other animals are somewhat different, but there is a kind of amœba, or crawling-cell, which grows on the top of stagnant water, that has a greater difference (Figs. 21, 22). Take a little of the scum that rises on ponds in hot weather, and put it under the microscope. In it you will find these jelly-lumps of a much larger size. They are from $1⁄1000$ to $1⁄100$ of an inch across, and move about by the same kind of queer-looking feet. But the border does not look at all the same. First, on the outside you see a clear, glassy-looking rim; inside of this is a thicker, darker ring filled with little grains—granules.

The centre of the cell is quite clear, and contains a thin space or vacuole, like that you saw in the yeast and green mould. The outer border or rim is called the ectosarc, which means outer flesh. The inner is called the endosarc, or inner flesh. Near the clear outer rim or ectosarc you will find the kernel or nucleus—a roundish, solid-looking little body which does not change its form. If you look closely you will see a small round, clear space in this outer rim or ectosarc which has motion, something like the beating of the heart. Indeed, by some it is thought to be the simplest form or beginning of a heart. It is called by a long name, the "contractile vesicle," or "contractile space." All amœbæ do not have this heart, nor do they all have a kernel. This contractile space or heart is very important, because it seems to be doing a work of its own. This is the first time we have found one part of a cell doing something entirely different from another part. The jelly or protoplasm of the yeast and the green mould is "maid-of-all-work." But the amœbæ family seem to be looking up in the world, and are trying to pattern after those establishments that keep a servant for each kind of work.

Inside of some of these large pond amœbæ you will often find the green protococcus-cells, little diatoms, desmids, and all kinds of cells