18 THE HUMAN BODY. 



substance of the already existing cell among its constituent 

 molecules. The new-formed molecules therefore contribute 

 to the growth of the cell not by superficial accretion, but by 

 interstitial deposit or intussusception.^ 



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 



FIG. 6. Diagrams illustrating direct cell-division, a, cell, body; 6, nucleus; 

 c, uucleoJus. 



equals the accumulation of new material, as it does in all the 

 cells of the Body when they attain a 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 

 superficial area, only increase as the square of the diameter. 

 The breaking down in the cell increases when its work 

 does, and so comes at last to equal the reception and con- 

 struction. 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 

 ^ases a comparatively simple process (Fig. 6). It was thought 

 that the nucleus, without any important structural change, 

 enlarged somewhat, 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 themselves around the new nuclei. The 

 nucleolus when present was supposed to divide before the 

 nucleus. Such a mode of cell multiplication 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 rearrangement of its 

 material: such cell division is named indirect, and the attend- 

 ant nuclear changes are known as the phenomena of karyoki- 

 nesis or mitosi* 



