THE CYCLE OF LIFE 397 



But as it is thidugli its surface that the cell is fed, aerated, 

 and purified, functional difficulties are bound to set in as 

 the increase of surface lags behind the increase of volume. 

 There is four times as much material to be kept aUve, but 

 there is not four times the surface by which to effect this. 

 A free-Hving cell, hke an Amceba, evades the functional 

 dilemma by flowing out into irregular processes, which 

 greatly increase the surface, making the cell like a country 

 with a much-indented coast-hne. But what ordinarily 

 happens is that when the cell has reached its hmit of growth, 

 the maximum safe size, it divides into two, halving its 

 volume and gaining new surface. As a general rationale, 

 appUcable mutatis mutandis to organs and organisms as 

 well as to cells, the suggestion thus briefly outhned certainly 

 helps towards an understanding of the hmit of growth. 

 Another important suggestion has been advanced by 

 Boveri and Richard Hertwig, that the hmit of growth is 

 in parb determined by the ratio of the amount of nuclear 

 material to the amount of cell-substance or cytoplasm. 

 When an animal grows larger this usually means that 

 its cells are multiplying, but it has been suggested (by 

 Plenk) that in lower animals of small size, such as Rotifers 

 and some Nematodes, an increase in the dimensions of the 

 cells plays an important part in growth. In the cells of 

 some animals with small eggs, such as lampreys and sala- 

 manders, there is some increase in the size of the elements, 

 and the same is true of very large ceUs in higher animals, 

 and of permianent elements hke ganghon-cells, muscle-cells, 

 and lens-fibres, which lose their power of division at a very 

 early stage. On the whole, however, cell-multiphcation 

 is the main factor in growth. The most characteristic 

 feature of a growing organism is that it is normally self- 



