Every organism composed of many cells was originally a single cell, and it becomes many-celled owing to the fact that the original cell propagates itself by self-division, and that the new individual cells originating in this manner remain together, and by division of labour form a community or a state. The forms and vital phenomena of all many-celled organisms are merely the effect or the expression of all the forms and vital phenomena of all the individual cells of which they are composed. The egg, from which most animals and plants are developed, is a simple cell. [Illustration: FIG. 2.—Propagation of a single-celled organism, Amœba sphærococcus, by self-division. _A._ The enclosed Amœba, a simple globular cell consisting of a lump of protoplasm (_c_), which contains a kernel (_b_) and a kernel speck (_a_), and is surrounded by a cell-membrane or capsule. _B._ The free Amœba, which has burst and left the cyst or cell-membrane. _C._ It begins to divide by its kernel forming two kernels, and by the cell-substance between the two becoming contracted. _D._ The division is completed by the cell-substance likewise falling into two halves (_Da_ and _Db_).] The single-celled organisms, that is, those which during life retain the form of a single cell, for example the Amœbæ, as a rule propagate themselves in the simplest way by self-division. This process differs from the previously described self-division of the Moneron only in the fact that at the commencement the firmer cell-kernel (nucleus) falls into two halves, by a pinching in at its middle. The two young kernels separate from each other and act now as two distinct centres of attraction upon the surrounding softer albuminous matter, that is, the cell-substance (protoplasma). By this process finally the latter also divides into two halves, and there now exist two new cells, which are like the mother cell. If the cell was surrounded by a membrane, this either does not divide at all, as in the case of egg-cleavage (Fig. 3, 4), or it passively follows the active pinching in of the protoplasm; or, lastly, every new cell exudes a new membrane for itself. [Illustration: FIG. 3.—Egg of a mammal (a simple cell). _a._ The small kernel speck or nucleolus (the so-called germ-spot of the egg). _b._ Kernel or nucleus (the so-called germ-bladder of the egg). _c._ Cell-substance or protoplasm (the so-called yolk of the egg). _d._ Cell-capsule or membrane (membrane of the yolk) of the egg; called in mammals, on account of its transparency, Membrana pellucida.]