RATIONALE OF CELL-DIVISION. 45 



inexplicable in terms of matter and motion, so long at least 

 as we do not know the secret of protoplasm. 



(4) On the other hand, Leuckart and Spencer have given 

 a general rationale of cell-division. Why do not cells grow 

 much larger, why do they almost always divide at a definite 

 limit of growth? Their answer is as follows. Suppose a young 

 cell has doubled its original mass, that means that there is 

 twice as much living matter to be kept alive. But the living 

 matter is fed, aerated, purified through its surface, which, 

 in growing spherical cells for instance, only increases as the 

 square of the radius, while the mass increases as the cube. 

 The surface growth always lags behind the increase of mass. 

 Therefore, when the cell has, let us say, quadrupled its 

 original mass, but by no means quadrupled its surface, 

 difficulties set in, waste begins to gain on repair, anabolism 

 loses some of its ascendancy over katabolism. At the limit 

 of growth, then, the cell divides, halving its mass and gaining 

 new surface. Of course surface may be increased by out- 

 flowing processes, just as it is by the lobes of many leaves, 

 and division may occur before the limit of growth is reached, 

 but as a general rationale, applicable to organs and bodies 

 as well as to cells, the suggestion of Leuckart and Spencer 

 is very helpful. 



Protoplasm. — I have spoken of the structure of living 

 matter or protoplasm in describing cells. I insert this 

 heading simply to emphasise anew that the morphological 

 as well as the physiological analysis passes from the organism 

 as a whole to its organs, thence to the tissues, thence to the 

 cells, and finally to the protoplasm itself. 



