1 82 The Study of Animal Life PART in 



bouring cells are often established ; and (4) of cell contents, 

 which can be chemically analysed, and which are products 

 of the vital activity rather than parts of the living substance, 

 such as pigment, fat, and glycogen or animal starch. 



The growth of all multicellular animals depends upon a 

 multiplication of the component cells. Like organisms, 

 cells have definite limits of growth which they rarely exceed ; 

 giants among the units are rare. When the limit of 

 growth is reached the cell divides. 



The necessity for this division has been partly explained 

 by Spencer and Leuckart. If you take a round lump of 

 dough, weighing an ounce, another of two ounces, a third 

 of four ounces, you obviously have three masses success- 

 ively doubled, but in doubling the mass you have not 

 doubled the surface. The mass increases as the cube, the 

 surface only as the square of the radius. Suppose these 

 lumps alive, the second has twice as much living matter as 

 the first, but not twice the surface. Yet it is through the 

 surface that the living matter is fed, aerated, and purified. 

 The unit will therefore get into physiological difficulties as 

 it grows bigger, because its increase of surface does not 

 keep pace with its increase of mass. Its waste tends to 

 exceed its repair, its expenditure gains on its income. 

 What are the alternatives ? It may go on growing and die 

 (but this is not likely), it may cease growing at the fit 

 limit, it may greatly increase its surface by outflowing 

 processes (which thus may be regarded as life-saving), or 

 it may divide. The last is the usual course. When the 

 unit has grown as large as it can conveniently grow, it 

 divides ; in other words, it reproduces at the limit of 

 growth, when processes of waste are gaining on those 

 of construction. By dividing, the mass is lessened, the 

 . surface increased, the life continued. 



But although we thus get a general rationale of cell- 

 division, we are not much nearer a conception of the 

 internal forces which operate when a cell divides ; for in 

 most cases the process is orderly and complex, and is 

 somehow governed by the behaviour of the nucleus. Few 

 results of the modern study of minute structure are more 



