36 GROWTH 



The seventh and last stage follows the age of sixty. 



The immediate mechanisms regulating these remarkable 

 changes with advancing age are not clear. We do, however, have 

 some theories which are very helpful in visualizing and "ex- 

 plaining" the processes and which, for this reason, deserve some 

 consideration. 



We may first consider the problem of growth in bulk. 

 Growth in bulk is largely due to the increase in the number of 

 cells in the body, and the problem is, therefore, reduced to a 

 consideration of the phenomenon of division of cells. Why do 

 cells divide? We have no ultimate answer to this question. We 

 can only explain this phenomenon in a figurative way by means 

 of an analogy. The fact is that when a young cell such as a 

 young bacterium or cell from the body of an animal is placed in 

 an appropriate nutrient medium, such as blood serum, it grows 

 in size. Why does it grow in size? Here the theory begins. Our 

 theory is that it grows in size because it is in an unstable equilib- 

 rium with the medium surrounding it, and growth is therefore 

 an expression of a tendency on the part of the cell to reach 

 stable equilibrium. The chemist, and the physicist, constantly 

 uses this idea of equilibrium in explaining what are known as 

 kinetic and dynamic processes, that is, processes involving 

 changes. As soon, however, as the cell reaches a certain size, it 

 becomes unstable with respect to its size, and this unstability is 

 made good by dividing. As soon as the cell divides it becomes 

 young again and unstable with respect to the medium surround- 

 ing it and so it increases in size again. The cell is thus always in 

 an unstable equilibrium either with respect to the medium or 

 with respect to its size, and our explanation is that this unsta- 

 bility is the immediate cause of increase in the number of cells 

 and consequently in the bulk of the body of the organism. 



An experimental illustration of this theory of unstable equi- 

 librium, but as related to a whole organism, was recently fur- 

 nished by Child. He found that a planarian, a species of flat- 

 worm, may be made, so to speak, to grow forward or backward, 



