50 THE ELEMENTS OF STRUCTURE. oe 
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 
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 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 
Ameeba, or an ovum, or any other cell, is really protoplasm. 
We are able to make negative statements,.eg, the yolk of 
an egg is not protoplasm, but we cannot make positive 
‘statements, or say, This is protoplasm, and nought else. 
