Within the cytoplasmic mass there is an 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 consists of a homogeneous ground-sub stance, or karyolymph, which is permeated by a meshwork that usually appears to consist of two substances, linin and chromatin, which are probably chemically closely related. Chromatin is the highly characteristic nuclear material which takes various forms during different phases of cell activity but generally gives the appearance of a network of tiny granules with one or more dense 'knots' of chromatin.
Frequently there are one or more conspicuous, round bodies within the nucleus known as nucleoli or singular nucleolus. Later it will be necessary to describe some of the important changes in chromatin arrangement that occur during various phases of cell activity, especially during cell division, but it is sufficient at this moment to emphasize that the nucleus is a differentiated area of the cell protoplasm which is the arena of the chromatin. Indeed, the nucleus probably represents the highest type of organization in the organism.