The majority of independent masses of protoplasm, and all highly organized cells, contain one or more nuclei in their substance. The nucleus is sharply marked off from the protoplasm, and is supposed to be surrounded by a special limiting membrane. Its presence can generally be made much more conspicuous by treating the cell with certain chemical reagents, notably dilute acids and various dyes. The nucleus is able to resist the action of dilute acetic acid better than the remainder of the cell, so that it stands out clearly, when the rest becomes transparent. Many staining agents, such as magenta (one of the aniline dyes), color the nucleus more quickly and deeply than the protoplasm. Although it is accredited with special independent movements that occur under certain circumstances, compared with the protoplasm it is not very contractile. It appears to be intimately associated with the vital phenomena of the cell, and may be said to control or initiate its most important activity, namely, its division. In the nuclear matrix, which is clear and homogeneous, may often be seen an irregular network, one point of which stands out more clearly, and is called the nucleolus. Remarkable changes in the arrangement of this network are seen in some cells to precede the division of the protaplasm. This is called karyokinesis.