Essay II.
The Chemistry of the Body.
I.
The The cell is usually very minute—indeed, absolutely invisible without a microscope, though in some cases it is a fair size. The whole yolk of an egg is a single cell until its minute nucleus, a speck on one side, starts dividing and it becomes several. By the time the chick is ready to be hatched there are millions.
Usually, however, a cell is small—just as much protoplasm as its still more minute nucleus can keep going; though here, again, one must be guarded, for there may be several nuclei instead of only one. The protoplasm on the external surface and around the nucleus is specialized into a more or less definite membrane. To this outer envelope are attached fine fibrils, which join up to a small body within the cell, called the centrosome, and by the lengthening and shortening of these its shape can be altered. The contents are fluids; so if the containing membrane is loosened in any direction, they tend to bulge out and form an excrescence, and in this way the cell is enabled to throw out limbs and surround particles of food, and, by relaxing the fibrils in one direction and contracting them in others, to crawl whither its chemical, thermal, or physical affinities direct. (See Diagram 1.)
Not particularly inspiring is the sight of life in its simplest form, but when a few millions of these cells
