58 THE ELEMENTS OF STRUCTURE 



We are far from being able to give even an approximate 

 account of the " mechanism " of cell division. The whole 

 process is vital, and cannot, at present at least, be re- 

 described in terms of matter and motion. 



On the other hand, Leuckart, Spencer, and Alexander 

 James 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 ? The answer is as 

 follows : — Suppose a young cell has doubled its original 

 volume, 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 volume, 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 the cell divides, halving its mass and gaining new 

 surface. It is true that the surface may be increased by out- 

 flowing processes, just as that of leaves by many lobes ; 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 above outUned is very 

 helpful. It is supported by an experiment due to Hart- 

 mann, who kept an Amoeba alive and healthy for over 

 four months, without any division, by amputating small 

 portions of the cytoplasm each day so that the size remained 

 constant. Amoebae which were not operated on divided 

 every second day. The ratio of the amount of nuclear 

 material in the cell to the amount of cytoplasmic material 

 seems also to have a determining influence upon cell division 

 (R. Hertwig). 



Protoplasm. — Morphological as well as 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. But although we may define protoplasm 

 as genuinely living matter — as " the physical basis of life " 

 — we cannot definitely say how much or what part of an 

 Amoeba, or an ovum, or any other cell, is really protoplasm. 



