=Unicellular Organisms.--=While such a structure as a tree or a horse is composed of countless millions of cells, on the other hand numerous organisms, both plant and animal, exist which consist of only one cell. Yet this cell is just as characteristically a cell as are the components of a complex animal or plant. It has the necessary parts, the cell body and the nucleus. Moreover it exhibits all of the fundamental activities of life, though in a simplified form, that a complex higher organism does. =Importance of Cell-Theory.--=This discovery that every living thing is a single cell or an aggregation of cooperating cells and cell-products is one of our most important biological generalizations because it has brought such a wide range of phenomena under a common point of view. In the first place, the structure of both plants and animals is reducible to a common fundamental unit of organization. Moreover, both physiological and pathological phenomena are more readily understood since we recognize that the functions of the body in health or disease are in large measure the result of the activities of the individual cells of the functioning part. Then again, the problems of embryological development have become much more sharply defined since it could be shown that the egg is a single cell and that it is through a series of divisions of this cell and subsequent changes in the new cells thus formed that the new organism is built up. And lastly, the problem of hereditary transmission has been rendered more definite and approachable by the discovery that the male germ is likewise a single cell, that fertilization of the egg is therefore the union of two cells, and that in consequence the mechanism of inheritance must be stowed away somehow in these two cells. =Heredity in Unicellular Forms.--=In unicellular animals one can readily see how it is possible for an individual always to give rise to its own kind. One of the simplest of the single-celled animals is the _Ameba_ (Fig. 4, p. 24).