Cellular structure begins, as has long been known, by the division of a cell into two, each of the parts then proceeding to grow by the assimilative power of protoplasm and in due time to divide in its turn. A mass of these cells is called ‘cellular tissue.’ The so-called ‘budding’ of a small cell from the side of the parent is, of course, simply a form of division. The process of division and redivision goes on, accompanied by a differentiation in the shape and function of the different cells or groups of cells which are formed, until the structure of the plant or animal is completed. In these operations the nucleus plays the principal part. The division of the cell is essentially the division of the nucleus. A detached portion of a cell which contains nothing of the nucleus can reproduce itself no more; it perishes. [Illustration: Fig. I. This illustration, which (by permission of The Macmillan Co.) I take from Wilson’s work, The Cell, is one of remarkable interest, for in it the microscope has caught, in a piece of actual tissue from the skin of the salamander, _Amblystoma_, three nuclei in different stages of mitotic division. Most of the nuclei, which are seen as large, roundish objects in their respective cells, show the chromatin in its ‘resting’ condition interspersed through the nucleus. The nucleus under _a_ shows the chromatin gathered into chromosomes. At _b_ the centrosomes with their astral figures (which can barely be detected) have been formed, the chromosomes have carried out their longitudinal division, and are being attracted half towards one centrosome and half towards the other. A little above this the process has been carried further, and the sides of the cell are beginning to contract, preparatory to forming two new ones. In Fig. 2 will be found a clear representation of the astral figures. _To face p. 40._] [Illustration: Fig. 2. The above illustration from Wilson’s The Cell shows in more or less diagrammatic form the stage of nuclear division in which the chromosomes, as yet undivided, have arranged themselves in the centre of the nucleus. The centrosomes with their astral figures have been formed, and have taken their places near each pole of the nucleus. The next stage is represented at _b_ in Fig. 1.]