50 THE ELEMENTS OF STRUCTURE. 



spermatozoon, and has for its net result the reduction of the 

 number of chromosomes to a half of the normal number. 



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 

 oiiginal 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 outlined is very 

 helpful. 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. 

 We are able to make negative statements, e.g. the yolk of 

 an egg is not protoplasm, but we cannot make positive 

 statements, or say, This is protoplasm, and nought else. 



