48 THE CELL 



nucleus of the elongated smooth muscle cell. It may also be modified 

 by intracellular pressure as, e.g., in the mucous cell and in the fat 

 cell. 



The position of the nucleus is usually near the center of the cell. 

 It may, however, be eccentric. Such eccentricity may be due to 

 pressure of cell contents as, e.g., in the mucous cell and in the fat cell. 

 Considered by earlier cytologists an unessential part of the cell, the 

 nucleus is now known to be most intimately associated with cellular 

 activities. It is not only essential to the carrying on of the ordinary 

 metabolic processes of the cell, but is an active agent in the phenomena 

 of mitosis, which in most cases determine cell reproduction. 



As a rule, each cell contains a single nucleus. Some cells contain 

 two nuclei (quite common in the liver cell, rare in the ovum and in the 

 nerve cell). A few cells contain many nuclei, e.g., the multinuclear 

 "giant" cells of the spleen, bone-marrow, and certain tumors. Some 

 cells, such as the human red blood cell and the respiratory epithelium, 

 are, in their mature condition, non-nucleated. All non-nucleated 

 cells, however, contained nuclei in the earlier stages of their develop- 

 ment. Non-nucleated cells, while capable of performing certain 

 functions, are wholly incapable of proliferation. The non-nucleated 

 condition must, therefore, be regarded as not only a condition of matur- 

 ity, but of actual senility, at least so far as reproductive powers are 

 concerned. 



In some of the lowest forms of animal life, the nuclear material 

 instead of being grouped to form a definite body or nucleus, is more 

 or less evenly distributed as granules through the cytoplasm. 



Chemically the nucleus is extremely complex, being composed of 

 the proteids nuclein, paranuclein, linin, paralinin, and amphipyrenin. 



Morphologically also the nucleus is complex, much of the apparent 

 structural differentiation being determined by the staining reactions 

 of the dift'erent elements when treated with certain anihne dyes. The 

 nuclear structures and their relations to the chemical constituents of 

 the nucleus are as follows: 



(a) The nuclear membrane (amphipyrenin). Tliis forms a limit- 

 ing membrane separating the nucleus from the cell protoplasm. It 

 is doubtful whether the nuclear membrane is different either chemic- 

 ally or morphologically from the nucleoreticulum. It is wanting in 

 some nuclei. When present it appears from its staining reactions to 

 be structurally continuous with, and chemically identical wdth, in some 

 cases, the liniii, in others, the chromatin of the intranuclear network. 



