In order to comprehend, in some measure, the transition from _Colonial Protozoa_ to _Metazoa_, it will be well for the reader to study a typical sponge. For a long time the Porifera (Sponges) were looked upon as compound Protozoa (colonial Protozoa), but while they are nearer the Protozoa than any of the other types of Metazoa, their position in the animal series is unquestionably among the Metazoa. The Sponge, like the rest of the Metazoa, develops from a fertilized egg by a process of cell multiplication, differentiation, gastrulation, etc. CELL REPRODUCTION BY MITOSIS. The multiplication of cells plays a part of such fundamental importance in Evolution, and therefore in Embryology and studies in Heredity, that it is necessary to study the subject somewhat in detail. It is a wonderful process, and is worthy of very careful attention. [Illustration: FIG. 8.—Diagram illustrating Mitosis. A, the cell commencing activity; B, C, D, phases in the formation of the spindle and the chromatin loops or V’s, also showing that the mother V’s have split into daughter V’s; D, the chromatin loops forming the equatorial plate, _chr_; E, F, G, separation of the daughter loops (daughter chromosomes) and their passage towards the poles of the spindle, thus forming daughter nuclei; H, I, division of the protoplasm so as to form two daughter cells; at, attraction sphere enclosing a centrosome; n m, nuclear membrane; chr., chromatin threads; p, protoplasm; c w, cell wall; sp, spindle.] The process by which one cell (a mother cell) divides into two cells (daughter cells) is called _mitosis_, and is inaugurated by the _centrosome_ (Fig. 8, A, _at_). The centrosome divides into _two_ centrosomes, which at first remain close together (Fig. 8, B, _sp_), and then gradually separate from one another. Each centrosome becomes the center of a system of fine achromatin fibers arranged round it in a radiating manner and forming what is called the _attraction sphere_; also, at the same time, a spindle-shaped bundle of achromatin fibers, called the _spindle_ (Fig. 8, B, sp), extends between the centrosomes. In the meantime, important changes have been taking place in the chromosomes (hereditary threads) of the nucleus. The chromosomes, which at first are arranged in an apparently inextricable tangle or network, frequently assume U-shaped or V-shaped forms (Fig. 8, C, _chr_), and the nuclear membrane disappears. Sooner or later each chromosome splits longitudinally into two daughter chromosomes, with which the achromatin fibers of the spindle become connected (Fig. 8, D). In this phase of mitosis the split V-shaped chromosomes form a single group called the _equatorial plate_ (chr), and extend across the axis of the spindle. It is to be observed from the diagrams in the figure, that one of the centrosomes has traveled to the opposite pole of the nucleus, thus causing the achromatin fibers of the spindle to extend across the original site of the nucleus.