The Nucleus.--To the earlier observers the protoplasm appeared to be a structureless, jelly-like mass containing granules and vacuoles; but closer acquaintance with it has shown that it is in reality very complex in structure as well as in chemical composition. It is by no means homogeneous; adjacent parts are different in properties and aptitudes. The nucleus, which is more readily seen than other cell elements, was shown to be of great importance in cell-life--to be a structure which takes the lead in cell division, and in general dominates the rest of the protoplasm. Chromosomes.--After dyes came into use for staining the protoplasm (1868), it became evident that certain parts of it stain deeply, while other parts stain faintly or not at all. This led to the recognition of protoplasm as made up of a densely staining portion called _chromatin_, and a faintly staining portion designated _achromatin_. This means of making different parts of protoplasm visible under the microscope led to important results, as when, in 1883, it was discovered that the nucleus contains a definite number of small (usually rod-shaped) bodies, which become evident during nuclear division, and play a wonderful part in that process. These bodies take the stain more deeply than other components of the nucleus, and are designated _chromosomes_. [Illustration: Fig. 77.--An Early Stage in the Development of the Egg of a Rock-Limpet. (After Conklin.)] Attention having been directed to these little bodies, continued observations showed that, although they vary in number--commonly from two to twenty-four--in different parts of animals and plants, they are, nevertheless, of the same number in all the cells of any particular plant or animal. As a conclusion to this kind of observation, it needs to be said that the chromosomes are regarded as the actual bearers of hereditary qualities. The chromosomes do not show in resting-stages of the nucleus; their substance is present, but is not aggregated into the form of chromosomes. [Illustration: Fig. 78.--Highly Magnified Tissue Cells from the Skin of a Salamander in an Active State of Growth. Dividing cells with chromosomes are shown at _a_, _b_, and _c_,. (After Wilson.)] Fig. 78 shows tissue cells, some of which are in the dividing and others in the resting-stage. The nuclei in process of division exhibit the rod-like chromosomes, as shown at _a_, _b_, and _c_. [Illustration: Fig. 79.--Diagram of the Chief Steps in Cell-division. (After Parker as altered from Fleming.)] Centrosome.--The discovery (1876) of a minute spot of deeply staining protoplasm, usually just outside the nuclear membrane, is another illustration of the complex structure of the cell. Although the centrosome, as this spot is called, has been heralded as a dynamic agent, there is not complete agreement as to its purpose, but its presence makes it necessary to include it in the definition of a cell.