Within the cytoplasmic mass of the cell there is a restricted area of clearly differentiated material, which typically has a rounded form, bounded by a membrane, so that it appears as a definite body of protoplasm called the nucleus.
The structural basis of the nucleus appears to be essentially similar to that of the cytoplasm the so-called LININ mesh work and KARYOLYMPH being comparable respectively to the granular meshwork and hyaline ground substance of the cytoplasm. But superimposed upon this, as it were, is the highly characteristic nuclear material, or Chromatin, which takes various forms during different phases of cell activity but generally, in a 'resting' cell, gives the appearance of a network of tiny granules with one or more dense 'knots' of chromatin (KARYOSOMES) .
Later we shall describe some of the important changes in chromatin arrangement, but it is sufficient at this time to emphasize that the nucleus is a differentiated area of the cell protoplasm which is the arena of the chromatin. Frequently there is a conspicuous round achromatic body within the nucleus known as the Nucleolus.
Cytoplasm and nucleus, looked at from the functional view point, represent a physiological division of labor within the confines of the cell. Experiments have shown that they are mutually necessary for cell life; the removal of the nucleus putting an end to anabolic processes assimilation, repair, and growth and thus leading rapidly to death. Accordingly the nucleus may be considered as the center of the synthetic activities of the cell, and the cytoplasm, if not as the area in which destructive processes are chiefly involved, at least as a neutral region which translocates material toward and away from the nucleus. All the evidence points to the nucleus as the " controlling center in cell activity, and hence a primary factor in growth, development, and transmission of specific qualities from cell to cell, and so from one generation to another."
