Section 51. This Is Shown in Figure Iii., Sheet 3, As It Would Appear

under the low power of the microscope. We have a mass of a clear, transparent, greyish substance called protoplasm, granular in places, and with a clearer border; within this is a denser portion called the nucleus, or endoplast (n.), which, under the microscope, by transmitted light, appear brighter, and within that a still denser spot, the nucleolus (ns.) or endoplastule. The protoplasm is more or less extensively excavated by fluid spaces, vacuoles; one clearer circular space or vacuole, which is invariably present, appears at intervals, enlarges gradually, and then vanishes abruptly, to reappear after a brief interval; this is called the contractile vacuole (c.v.). The amoeba is constantly changing its shape, whence its older name of the Proteus animalcule, thrusting out masses of its substance in one direction, and withdrawing from another, and hence slowly creeping about. These thrust-out parts, in its outline, are called pseudopodia (ps.). By means of them it gradually creeps round and encloses its food. Little particles of nutritive matter are usually to be detected in the homogeneous protoplasm of its body; commonly these are surrounded by a drop of water taken in with them, and the drop of water is then called a food vacuole. The process of taking in food is called ingestion. The amoeba, in all probability, performs essentially the same chemical process as we have summarised in Sections 10, 11, 12; it ingests food, digests it in the food vacuoles and builds it up into its body protoplasm, to undergo kataboly and furnish the force of its motion-- the contractile vacuole, is probably respiratory and perhaps excretory, accumulating and then, by its "systole" (compare Section 44), forcing out of its body, the water, carbon dioxide, urea, and other katastases, which are formed concomitantly with its activity. The amoeba reproduces itself in the simplest way; the nucleus occasionally divides into two portions and a widening fissure in the protoplasm of the animal's body separates one from the other. It is impossible to say that one is the parent cell, and the other the offspring; the amoeba we merely perceive, was one and is now two. It is curious to note, therefore, that the amoeba is, in a sense, immortal-- that the living nucleus of one of these minute creatures that we examine to-day under a microscope may have conceivably drawn, out an unbroken thread of life since the remotest epochs of the world's history. Although no sexual intercourse can be observed, there is reason to believe that a process of supposed "cannabalism," in which a larger amoeba may occasionally engulph a smaller one, is really a conjugative reproductive process, and followed by increased vitality and division.