THE CELL 17 



The chromatin fibres vary in their arrangement in different 

 cells. Usually they form a network, but occasionally they 

 are disposed as a continuous skein. In nuclei, with the former 

 arrangement of fibres, swellings may be observed where the 

 fibres unite with one another the nodal swellings, or karyo- 

 somes, distinct from the nucleolus. The resting nucleus appears 

 to be surrounded by a distinct mwlear membrane, which is, how- 

 ever, probably really a basket-like interlacement of the fibres 

 at the periphery. 



Between the fibres is (b) a more fluid material which may 

 be called the nuclear plasma or karyoplasrn. Digestion in the 

 stomach removes the nuclear plasma, but leaves the network 

 unacted upon. 



(2) Functions. The part taken by the nucleus in the general 

 life of the cell is not yet fully understood. 1st. It exercises 

 an influence on the nutritive processes, since it has been 

 observed in certain of the large cells in lower organisms that 

 a piece of the protoplasm detached from the nucleus ceases to 

 grow, and, after a time, dies. Important interchanges of material 

 go on between the nucleus and the protoplasm. 2nd. It is 

 the great reproductive organ of the cell, probably playing an 

 important part in transmitting inherited characters (see p. 194). 



Reproduction of Cells 



Cells do not go on growing indefinitely. When they reach 

 a certain size they generally either divide, to form two new 

 cells, or they die and undergo degenerative changes. The 

 reason of this is possibly to be found in the well-known physical 

 fact that, as a sphere increases in size, the mass increases more 

 rapidly than the periphery. Hence, as a cell becomes larger 

 and larger, the surface for nourishment becomes smaller and 

 smaller in relationship to the mass of material to be nourished. 

 Probably the altered metabolism so produced sets up the changes 

 which lead to the division of the cell. These changes have now 

 been very carefully studied in a large number of cells, and it 

 has been shown that the nucleus generally takes a most 

 important part in division. 



Mitosis. In a cell about to divide, the first change is a 

 general enlargement of the nucleus. At the same time the 

 centrosome becomes double, and the two portions travel from 



