Footnote 25: Amoebae _may_ be obtained from the hay infusion, from the dead leaves in the bottom of small pools, from the same source in fresh-water aquaria, from the roots of duckweed or other small water plants, or from green algae growing in quiet localities. No _sure_ method of obtaining them can be given. Although but a single cell, still the amoeba appears to be aware of the existence of food when it is near at hand. Food may be taken into the body at any point, the semifluid protoplasm simply rolling over and engulfing the food material. Within the body, as in the paramoecium, the food becomes inclosed within a fluid space or vacuole. The protoplasm has the power to take out such material as it can use to form new protoplasm or give energy. Circulation of food material is accomplished by the constant streaming of the protoplasm within the cell. [Illustration: Amoeba, showing the changes which take place during division of the cell. The dark body in each figure is the nucleus; the transparent circle, the contractile vacuole; the large granular masses, the food vacuoles. Much magnified.] The cell absorbs oxygen from the water by osmosis through its delicate membrane, giving up carbon dioxide in return. Thus the cell "breathes" through any part of its body covering. Waste nitrogenous products formed within the cell when work is done are passed out by means of the contractile vacuole. The amoeba, like other one-celled organisms, reproduces by the process of fission. A single cell divides by splitting into two others, each of which resembles the parent cell, except that they are of less bulk. When these become the size of the parent amoeba, they each in turn divide. This is a kind of asexual reproduction. When conditions unfavorable for life come, the amoeba, like some one-celled plants, encysts itself within a membranous wall. In this condition it may become dried and be blown through the air. Upon return to a favorable environment, it begins life again, as before. In this respect it resembles the spore of a plant. [Illustration: Vorticella. _e_, gullet; _n_, nucleus; _cv_, contractile vacuole; _a_, axis; _s_, sheath; _fv_, food vacuole. (From Herrick'_s General Zoology_.)] The Cell as a Unit.--In the daily life of a one-celled animal we find the single cell performing all the general activities which we shall later find the many-celled animal is able to perform. In the amoeba no definite parts of the cell appear to be set off to perform certain functions; but any part of the cell can take in food, can absorb oxygen, can change the food into protoplasm, and excrete the waste material. The single cell is, in fact, an organism able to carry on the business of living almost as effectually as a very complex animal.