CH. II. J CELL DIVISION 15 



Cell Division. 



A cell multiplies by dividing into two; each remains awhile 

 in the non-dividing condition, but later it grows and subdivides, and 

 the process may be repeated indefinitely. 



The supreme importance of the cell, the growth of the body from 

 cells, and the fact that cells are the living units of the organism, 

 were first established in the vegetable world by Schleiden, and 

 extended to the animal kingdom by Theodor Schwann. The ideas 

 of physiologists depending on this idea are grouped together as 

 cellular physiology, which under the guidance of Virchow was ex- 

 tended to pathology also: Virchow expressed the doctrine now so 

 familiar as to be almost a truism in the terse phrase omnis cellula e 

 cellula (every cell from a cell). 



The division of a cell is preceded by division of its nucleus. 

 Nuclear division may be either (1) simple or direct, which consists in 

 the simple exact division of the nucleus into two equal parts by con- 

 striction in the centre, which may have been preceded by division of 

 the nucleolus ; or (2) indirect, which consists in a series of changes 

 which goes on in the arrangement of the nuclear reticulum, resulting 

 in the exact division of the chromatic fibres into two parts, which 

 form the chromoplasm of the daughter nuclei. 



The changes in the nucleus during indirect division constitute 

 karyokinesis (Kapvov, a kernel), or mitosis OU/TO?, a thread), and 

 direct division is called amitotic or akinetic (Kivya-is, movement). It 

 is now believed that the mitotic nuclear division is all but, though 

 not quite, universal. Somewhat different accounts of the stages of 

 the nuclear division have been given by different authorities, accord- 

 ing to the kind of cell in which the nuclear changes have been 

 studied ; but, speaking generally, the process may be divided into the 

 following stages : 



1. The non-dividing nucleus (fig. 16). 



Node of network ^ ,. . ..... , ,^ 



_.*-. Nuclear membrane. 



.Nucleolus. 



Node of network l-^^^^p^xfC^T^-^- Nuclear matrix. 



( Nuclear network. 



FIG. 16. The non-dividing nucleus. (Waldeyer.) 



2. The spirem or skein stage : the nucleoli dissolve, and the 

 nuclear filaments form loops which run from one pole of the nucleus 



