Page:Popular Science Monthly Volume 6.djvu/360

346 that are working there. When I watched the ducks swimming in the old trough, little did I think of the noiseless hands that were building up those velvety-green walls, or of the unseen and unnumbered fleet of boats sculling through the water.

Young folks have a great fancy for "bloody stories;" so I am going to tell you not exactly a "bloody story," but a story about blood, and you need not be alarmed, for before I finish you will find there is nothing in blood to alarm any one, but a great deal that is useful, curious, and beautiful. If you prick your finger with a needle, and squeeze out a drop of blood and place it under the microscope, you will be astonished at what you see (Fig. 14). You can hardly believe that a drop of blood contains so many curiosities. First you observe a whole lot of little reddish-looking bodies, and among these a number of larger transparent bodies, which look like minute splashes of light-colored jelly. It is about these jelly-like bodies I am going to talk with you. If

you keep your eye on one of them, you see that it continually changes its form, and that it has a slow, crawling kind of motion; and, if you try to make a drawing of it on paper, your picture will never be twice alike (Figs. 15, 16). It puts out something from one side which looks like a foot; then it draws in this foot, and puts out another at the other side, as if trying to find a soft place to walk upon. Sometimes it puts out several of these feet at one time. This little jelly-splash appears to use its feet as we use ours, to walk with, though you see it gets on quite slowly and awkwardly. Its foot is called a pseudopodium, which means false foot. These little bodies have a very suitable name—amœbæ, and the word means changing. This name was given to them, no doubt, because they are constantly changing their form. The amoeba, or blood-cell, is larger than the still protococcus, or mould of the paling, and not quite as large as the moving protococcus, or green water-mould. It is usually about 1/2500 of an inch in breadth. It does not possess the cellulose or woody sac, like the little protococcus houses. It is more like the pear-shaped protococcus boatmen. Its wall is just the hardened outer layer of the jelly or protoplasm. It has no thin