PHYSIOLOGY OF CELLS 69 



reactions in which the chromatin is involved occur with the chromatin 

 still in the nucleus. What soluble substances may pass back and forth 

 between nucleus and cytoplasm is completely unknown. Chromatin 

 is a very complex matter. There is evidence, which is briefly mentioned 

 in the chapter on Genetics, that the chromatin of a single cell may com- 

 prise hundreds of different things. What these things are is a matter 

 of speculation, but that they are responsible for as many different 

 activities in the cells has been pretty well determined. Furthermore, 

 there is a great variety of other substances in the nucleus and in the cyto- 

 plasm. The complexity of the protoplasmic mixture is great enough to 

 account for the great number of activities that have been demonstrated 

 in cells. 



Deferred Subjects. — Numerous activities of cells reveal their full 

 significance only in relation to the activities of cells around them, or 

 to the processes going on in distant organs. Discussion of these must 

 await the further development of the idea of complexity of structure in 

 the succeeding chapters. Complexity of structure, resulting in com- 

 plexity of function, is a consequence of the adherence of cells after divi- 

 sion and their morphological differentiation. Cell division, which is 

 itself one of the characteristic activities of living matter, is described 

 in the next chapter, and differentiation in the one following. 



References 



Bayliss, W. M. Principles of General Physiology. 

 Ganong, W. F. The Living Plant. 

 LoEB, J. The Dynamics of Living Matter. 

 Taylor, W. W. The Chemistry of Colloids. 

 Verworn, M. General Physiology. 



