THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 19 



Cell Division. The increase of size, which may be brought about 

 in the above manner, is not indefinite, but is limited in two ways. 

 Alongside of the formation and deposit of new material there 

 occurs always in the living cell a breaking down and elimination 

 of the old; and when this process equals the accumulation of new 

 material, as it does in all the cells of the Body when they attain a 



Fio. 6. Diagram illustrating direct cell division, a, cell-body; 6, nucleus; 

 e, nucleolus. 



certain size, growth of course ceases. In fact the work of the cell 

 increases as its mass, and therefore as the cube of its diameter; 

 while the receptive powers, dependent primarily upon the super- 

 ficial area, only increase as the square of the diameter. The break- 

 ing down in the cell increases when its work does, and so comes 

 at last to equal the reception and construction. The second 

 limitation to indefinite growth is connected with the power of the 

 cell to give rise to new cells by division. 



Until recently it was believed that cell division was in all cases a 

 comparatively simple process (Fig. 6). It was thought that the 

 nucleus, without any important structural change, enlarged some- 

 what, became elongated, and then divided by simple constriction 

 into two equal parts, forming two smaller daughter nuclei; and 

 that the rest of the cell then divided, its halves arranging them- 

 selves around the new nuclei. The nucleolus when present was 

 supposed to divide before the nucleus. Such a mode of cell multi- 

 plication is known as direct division: it possibly occurs in some 

 cases, but in the great majority of cells division is preceded by 

 marked changes in the structure of the nucleus and by a rearrange- 

 ment of its material: such cell division is named indirect, and the 

 attendant nuclear changes are known as the phenomena of 

 karyokinesis or mitosis. 



Indirect, Karyokinetic or Mitotic Cell Division. Before at- 

 tempting to describe the phenomena of indirect cell divisions it is 

 necessary to give some account of the structure of a typical primi- 

 tive cell as made out in specimens carefully prepared and studied 



