io8 The Science of Life. 



hope that even cell-division will yield to physiological 

 analysis, that is to say, that some proximate solution 

 will be arrived at. 



A general rationale of why cell-division should take 

 place seems to have been suggested independently by 

 Leuckart, Spencer, and Alexander James. It is often 

 referred to as the Leuckart-Spencer principle. Why do 

 not cells go on growing larger and larger? why do they 

 almost always divide at a limit of growth more or less 

 definite for each kind of cell in given surroundings? 

 The answer is as follows: Suppose a young cell, 

 spherical in form, to have doubled its original mass by 

 growth, that means that there is twice as much living 

 material to be kept alive. But the living material is fed, 

 aerated, and purified through the cell -surface, which 

 only increases as the square of the radius, while the 

 mass increases as the cube. The extension of surface 

 must lag behind the increase of mass. Therefore when 

 the cell has, let us say, quadrupled its original mass, 

 but by no means quadrupled its surface, physiological 

 difficulties set in, the normal ratio between repair and 

 waste, construction and disruption, is seriously dis- 

 turbed. At the limit of growth, then, the cell divides, 

 halving its mass, and gaining new surface. It is true 

 that surface may also be increased by outflowing pro- 

 cesses, just as that of a leaf is by the formation of many 

 lobes ; and it is true that division may occur before the 

 limit of growth is reached, but as a general rationale, 

 quite different from physiological analysis, the Leuckart- 

 Spencer principle seems a useful suggestion, and it is 

 applicable to organs and to bodies as well as to cells. 



An interesting suggestion in regard to the forms 

 and phases of cell-life is due to Prof. Patrick Geddes. 

 The Ceil- It may be called the conception of a cell- 

 cycle, cycle. 



(i) In the life-history of one of the simplest organisms 

 ever described Haeckel's Protomyxa there are four 

 chapters. In one chapter, the organism is encysted 

 and breaks up into spores. These spores escape as 

 minute lashed (flagellate) units. As they feed, they sink 

 into an amoeboid form, like minute irregular drops of 



