Figures 3 and 4 show that, while these changes are being produced, each of the two centrosomes is surrounded by stellate rays. Some of these rays extending in the direction of the chromosomes, become attached to one of their extremities and draw it toward the corresponding centrosome (Fig. 7). Thus around each centrosome are grouped as many chromosomes as the mother cell possessed itself (Fig. 8). Simultaneously, the cell enlarges and its protoplasm commences to become indented at each end of the diameter previously formed by the chromosomes. From this moment the nuclear liquid concentrates itself around each of the groups of chromosomes, the rays disappear and the cell divides into two halves, each containing a group of chromosomes (Fig. 9); the indentation increases so as to form a partition across the protoplasm. The chromosomes then form a new meshwork of nuclear chromatin, and we have then two cells each with a nucleus and a centrosome like the mother cell (Fig. 10). This is what takes place in the reproduction of all cells of the animal and vegetable kingdoms. In the simplest unicellular organisms which are known fission constitutes the only means of reproduction. In the complicated organisms of the higher plants and animals each cell divides in the manner indicated above, both in the embryonic period and later on during the development of each of the organs which forms the organism. This fact shows more than any other the intimate relationship which connects all living organisms. The most remarkable thing, perhaps, is the almost mathematical division of the chromosomes into two halves, a division which results in the equal distribution of their substance through the whole organism. We shall return to this point later on.