530 GENERAL PHYSIOLOGY 



in sufficient quantity all substances necessary to its life, and let us 

 assume that the cell grows. With the increasing size of the cell 

 the relation of surface to mass will gradually change ; according to 

 known mathematical laws the former will grow in comparison with 

 the latter in the proportion of the square to the cube. In other 

 words, the smaller the cell, the greater is the surface in proportion 

 to the mass ; and the more the cell grows, the less does the surface 

 grow in proportion to the mass. 



This simple fact is of fundamental importance. This becomes 

 at once clear, when it is realised that the individual parts of the 

 cell-body are in close metabolic relations to one another and to the 

 external world. As regards the food -stuffs and the oxygen received 

 from the outside, the more the cell grows, the more a disproportion 

 between the external and the internal layers of the cell-body will 

 come about ; for, since the surface through which the food is taken 

 in increases less than the mass of the cell-body, the time will come 

 when the ingested food is no longer sufficient for the whole body, 

 and the result of this must become evident in the fact that the 

 internal cell-layers are too little nourished in comparison with the 

 external. While nutrition goes on rapidly and richly in the latter, 

 in the former it proceeds more slowly and more sparingly. This 

 will affect not only the protoplasm, but also the nucleus. The 

 nucleus will receive fewer substances from the outside, if the proto- 

 plasmic layer surrounding it becomes gradually thicker, than if it 

 is thin. But, vice versa, the external layers of the cell will become 

 provided with nuclear substances much less richly than the internal. 

 In brief, with the close relationship of the individual parts of the 

 cell the metabolism must undergo profound changes, which increase 

 the more, the more the cell grows. Hence, so long as the cell con- 

 tinues to grow, at no time is its metabolism exactly the same as at 

 the preceding and the following intervals. 



This important consequence from the fact of growth contains 

 within itself the principle of all development, i.e., with the close 

 metabolic relations that exist between the individual constituents 

 of the cell and of the medium, the fact of growth is alone completely 

 sufficient to lead and must lead to all the changes that are termed 

 " development. " 



It follows from these considerations that the cell can never 

 surpass a certain size ; for, if the disturbance of metabolism that 

 arises because of the increasing disproportion between the more 

 superficial and the deeper layers, has reached a certain extent, the 

 cell can no longer continue living in its existing form. Thus the 

 remarkable fact is explained very simply, that no cells of constant 

 form are known that are larger than a few millimetres in diameter ; 

 and thus we are made to understand why the development of 

 large organisms is only possible by the arrangement of the living 

 substance into an aggregate of small cells, instead of into a single 



