GROWTH AND REPRODUCTION. 



21 



waste secures the growth of the cell. Then a nemesis of grow- 

 ing wealth begins. The increase of surface is necessarily 

 disproportionate to that of contents, and so there is less 

 opportunity for nutrition, respiration, and excretion. Waste 

 thus gains upon, overtakes, balances, and threatens to exceed 

 repair. Suppose a cell to have become as big as it can well be, 

 a number of alternatives are possible. Growth may cease, and 

 a balance be struck ; or the form of the unit may be altered, 

 and surface gained by flattening out, or very frequently by 

 outflowing processes. On the other hand, waste may continue 

 on the increase, and bring about dissolution or death ; while 

 closely akin to this, there is the most frequent alternative, that 

 the cell divide, halve its mass, gain new surface, and restore the 

 balance. Here, in fact, the famous law of Malthus holds good. 

 § 3. Cell-Division. — What usually occurs, then, at the maxi- 

 mum or limit of growth, is that the cell divides. This, in its 

 simplest forms, is rough enough to suggest rupture or overflow ; 

 but in the vast majority of cases it is an orderly and definite 



Diagram of the changes in the nucleus during cell-division : — coil stage 

 («),_ the formation of a douhle star (/', c, d), and the recession of the 

 divided chromatin elements to opposite poles (t-) to form the daughter- 

 nuclei (/") of the two daughter-cells. — From Hatschek, after 

 Flemmincr. 



