Nucleus

Nucleus, a structure occurring in almost every cell (q.v.), whether animal or vegetable, and now recognised as of the highest importance in the life of the cell. The division of the protoplasm in cell division is generally preceded by that of the nucleus, and it has been observed that, whilst a mass of protoplasm escaping from a broken cell of the alga Vaucheria, if it contain a nucleus, will form a cell wall and grow into a new plant, if it contain no nucleus it will simply decompose. Every nucleus apparently originates from the division of a preexisting one, and, whether animal or vegetable, goes through an identical series of changes in itself dividing. It is denser than the protoplasm (q.v.), contains more phosphorus, and is more deeply stained by carmine. It has a limiting membrane or nuclear wall of a reticulate character, with more fluid contents, and contains two or more rounded bodies known as nucleoli. Though rounded when the cell is not dividing, it becomes spindle-shaped, and is then constricted into two spindle-shaped parts before dividing, whilst the granules of the protoplasm group themselves in plume-like lines radiating from its extremities like iron-filings in the magnetic field. These groupings are known as haryo Hnesis or haryolitic figures. The term nucleus was also applied to the body of the ovule (q.v.) as distinct from its coats; but, to avoid ambiguity, this structure is now preferably known as the nucellus or tercine. Every animal begins life as a simple cell [Embryology and Cell], while some animals (all in the phylum Protozoa) remain permanently in this form. One part of the cell is always separated from the remainder as a compact dark spot, which is known as the nucleus. In its simplest form this is merely a rounded spot which is homogeneous in composition; the material of which it is composed is called nuclein or chromatin. The latter name is given to it from its property of absorbing staining reagents much more readily than the rest of the cell. In most cases the nucleus is constructed on a more complex plan. The nuclein is arranged into a long thread, which may be twisted into an intricate series of knots or woven into a reticulate meshwork. The spaces between the nuclein threads are filled with a fluid material known as caryoplasma or achromatin, the latter name being derived from the fact that it is not readily coloured by staining reagents. The nucleus plays an important part in the division of the cells in their multiplication. There are certain bodies resembling cells in all respects except in the absence of a nucleus. Such are known as cytodes. The term nucleus is also used in zoology for a few other structures, such as the digestive organs of salpa.