A cell (Fig. 1) is a _nucleated lump of protoplasm_, or cytoplasm, and most often of microscopic size and more or less covered on its exterior by, and holding in its interior, various products and formations resulting from its activity, which are called _metaplasm_. Since the protoplasm of a cell, under the microscope, presents a superficial resemblance to a minute speck of that jelly-like substance (albumen) which forms the white of an egg, it is often called an albumenoid substance. But it is very misleading to use such an expression, for protoplasm is not a single chemical substance of great complexity; but it is rather composed of a large number of different chemical substances of great complexity. Many of these substances, it is true, are albumenoid in character. The same is true as to the chemical complexity of the nucleus, which is a physically and chemically differentiated part of the protoplasm. The protoplasm contains certain globulins, and also albumins and peptones; it also contains large quantities of nucleo-albumins, with other substances. The nucleus not only contains these same substances, but also nuclein and nucleo-proteids. It is important to state that nuclein consists of an albumin and nucleic acid. [Illustration: FIG. 1.—Diagram of a Cell, highly magnified.] The protoplasm, structurally, is made up of threads forming a complex, sponge-like substance, or reticulum, called _spongioplasm_; and in the meshes of the spongioplasm is a more or less fluid-like substance known as _hyaloplasm_: suspended in the hyaloplasm are various kinds of living bodies known as _plastids_, besides various products resulting from the activity of the protoplasm and which are designated _metaplasm_. In many cells the protoplasm has formed on its periphery a layer of metaplasm which is frequently called a cell-wall. This cell-wall prevents amœboid movements of the protoplasm, and a cell possessing it is said to be _encysted_. In many cells, especially vegetable ones, will also be observed clear spaces termed _vacuoles_. These vacuoles contain water with various chemical substances held in solution, which serve the purpose chiefly of food-reservoirs. The nucleus also is formed of threads called _nuclear or chromatin threads_ (_chromosomes_), the interstices of which are filled with hyaloplasm or _achromatin_. In the nucleus can also be observed the nucleolus. The protoplasmic and nuclear threads show various structural modifications in different regions and under different physiological states of the cell. As will be observed later on, the nuclear threads are of special interest to the student of heredity. They may in one phase of cell-activity look like one thread forming an inextricable network, while in other phases they may look like thick, short, distinct rods.