Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: 2. Form and Structure of the Cell.—Cells exhibit immense variety in size, in form, in structural complexity, and particularly in functional activity. A few may be large enough to be visible to the naked eye, but most are far too small to be seen without the aid of magnifying lenses. In its essential features a cell (Fig. 1) consists of a minute globule of matter, the cell body, containing in its centre a smaller body called the nucleus. A more or less clearly denned membrane, the cell wall, may surround the body of the cell. When a cell is stained with aniline dyes the nucleus takes the stain more strongly than the cell Cell wall or limiting membrane Nuclear membrane Nucleus Cell substance or cytoplasm Fio. 1.—Diagram of a cell. body and appears clearly and sharply defined, demonstrating its difference in chemical composition from the remaining cell substance. The nucleus is believed to be the most important element in the cell structure. It contains a special substance, peculiar to living matter, known as "chromatin" or "chromo- plasm," which appears to play the most important role in the cell activities. The substance of the cell body is called "cytoplasm." It may be smooth or granular in appearance, and sometimes has the suggestion of an intracellular network. The consistence of the cell substance is probably that of a semifluid or thin jelly. The solid part of plants and animal bodies are not generally regarded as part of the living cell substance, but as inert material built up by the chemical activities of the cell. Many cells, particularly among the single-celled organisms, have chapter{Section 4special (structural appendages to facilitate their motion or for other uses. 3. The Activities of the Cell: (1) Movement.—Many cells have the power of motio...