As we have said, the single cell is the physical centre, or parent, of every living form. It contains what is known as the _nucleus_, or kernel, which seems to be more highly organized than the rest of the material of the cell--it may be considered as the "brain" of the cell, if you wish to use your imagination a little. The single cell reproduces itself by growth and division, or separation. Each cell manifests the functions of life, whether it be a single-celled creature, or a cell which with billions of others, goes to make up a higher form. It feels, feeds, grows, and reproduces itself. In the single-celled creature, the one cell performs all of the functions, of course. But as the forms become more complex, the many cells composing a form perform certain functions which are allotted to it, the division of labor resulting in a higher manifestation. This is true not only in the case of animal forms, but also in the case of plant forms. The cells in the bone, muscle, nerve-tissue and blood of the animal differ according to their offices; and the same is true in the cells in the sap, stem, root, leaf, seed and flower of the plant. As we have said, the cells multiply by division, after a period of growth. The cell grows by material taken into its substance, as food. When sufficient food has been partaken, and enough new material accumulated to cause the cell to attain a certain size, then it divides, or separates into two cells, the division being equal, and the point of cleavage being at the kernel or nucleus. As the two parts separate, the protoplasm _of_ each groups itself around its nucleus, and two living forms exist where there was but one a moment before. And then each of the two cells proceed to grow rapidly, and then separate, and so on to the end, each cell multiplying into millions, as time passes. Ascending in the scale, we next find the living forms composed of cell-groups. These cell-groups are formed by single cells dividing, and then subdividing, but instead of passing on their way they group themselves in clusters, or masses. There are millions of forms of these cell-group creatures, among which we find the sponges, polyps, etc.