We find here already the special and fundamental act of conjugation, that is the fusion of two cells into one, which serves to strengthen reproduction. This act, common to all living things including man, shows us that continuation of life is only possible when from time to time different elements, that is elements which have been exposed to different influences, combine together. If this conjugation is prevented and life is allowed to continue indefinitely by means of fission or by budding (_vide infra_), there results a progressive weakening and degeneration which leads to the disappearance of the whole group thus reproduced. It is necessary to explain here the results of recent scientific work on the intimate phenomena of cell-division, for they are closely allied to those of fecundation. The nucleus of an ordinary cell presents itself in the form of a nearly spherical vesicle. Delicate methods of staining have shown that the nucleus encloses several round nucleolar corpuscles, and also a reticulum which is attached to its membrane and spreads through its whole substance. The liquid part of the nucleus fills the meshes of this reticular tissue, which stains easily and for this reason is named _chromatin_. The phenomena of cell division in well-developed cells with nuclei is termed _mitosis_. Certain lower forms of cells exist in which the nucleus is not well differentiated. Mitosis begins in the nucleus (Plate I). Figure 1 represents the cell before division has commenced. In the protoplasm, by the side of the nucleus, is formed a small corpuscle (_c_) which is called the _centrosome_. The nucleus itself is marked _b_. When the cell commences to divide, the meshes of the network of chromatin contract and the centrosome divides into two parts (Fig. 2). Shortly afterward the particles of chromatin concentrate in the form of convoluted rods called _chromosomes_ (Figs. 3 and 4). The number of these varies according to the species of organism, but remains constant for each animal or vegetable species. At the same time the two centrosomes separate from each other on each side of the nucleus. The chromosomes then become shorter and thicker while the nucleus is completely dissolved in the protoplasm of the cell, and its membrane disappears (Fig. 4). Directly afterwards the chromosomes arrange themselves regularly in line, like soldiers at drill, following one of the larger diameters of the cell, and forming a barrier between the two centrosomes (Fig. 5). Each of the chromosomes then divides into two parallel halves of equal thickness (Fig. 6).