IV] AND STRUCTURE OF THE CELL 333 



gradual alteration; and changes in the outward configuration of 

 the system are bound, consequently, to take place. 



Perhaps we may simplify the case still more. We have learned 

 many things about cell-division, but we do not know much in the 

 end. We have dealt, perhaps, with too many related phenomena, 

 and failed because we tried to combine and account for them all. 

 A physical problem, still more a mathematical one, wants reducing 

 to its simplest terms, and Dr Rashevsky has simplified and general- 

 ised the problem of cell-division (or division of a drop) in a series of 

 papers, which still outrun by far the elementary mathematics of 

 this book. If we cannot follow^ him in all he does, we may find 

 useful lessons in his way of doing it. Cells are of many kinds; they 

 differ in size and shape, in visible structure and chemical com- 

 position. Most have a nucleus, some few have none; most need 

 oxygen, some few do not; some metabolise in one way, some in 

 another. What small residuum of properties remains common to 

 them all? A living cell is a little fluid (or semi-fluid) system, in 

 which work is being done, physical forces are in operation and 

 chemical changes are going on. It is in such intimate relation with 

 the world outside — its own milieu interne with the great ynilieu 

 externe — that substances are continually entering the cell, some to 

 remain there and contribute to its growth, some to pass out again 

 with loss of energy and metabolic change. The picture seems 

 simplicity itself, but it is less simple than it looks. For on either 

 side of the boundary- wall, both in the adjacent medium and in the 

 living protoplasm within, there will be no uniformity, but only 

 degrees of activity, and gradients of concentration. Substances 

 which are being absorbed and consumed will diminish from periphery 

 to centre; those which are diffusing outwards have their greatest 

 concentration near the centre, decrease towards the periphery, and 

 diminish further with increasing distance in the near neighbourhood of 

 the system. Size, shape, diffusibility, permeability, chemical properties 

 of this and that, may affect the gradients, but in the living cell the 

 interchanges are always going on, and the gradients are always there *. 



* Outward diffusion makes one of the many contrasts between cell-growth and 

 crystal-growth. But the diffusion^gradients round a growing crystal are far more 

 complicated than was once supposed. Cf. W. F. Berg, Crystal growth from 

 solutions, Proc. R.8. (A), clxiv, pp. 79-95, 1938. 



