170 The Unity of the Organism 



ous processes, dendrites and axones, do actually grow out 

 on nerve cells and bring cells into connection with one 

 another and with receptor and effector cells, and that a 

 functional coordination is thus finally reached does not 

 exist in any way or degree in the early stages. Proof that 

 the completed stages of neural integration cannot be ac- 

 complished without the production of cell-growths which 

 put cells into connection with one another is a very different 

 thing from proof that these cells were once wholly isolated 

 from one another, and that the outgrowths which establish 

 the final connections were initiated by impulses which origi- 

 nated wholly within the cells ; as, by way of illustration, the 

 branches of two young trees standing not far apart might 

 come into contact as the trees increased in size. It is a 

 matter of elementary knowledge of animal development that 

 in this sense the cells of the nervous system are never iso- 

 lated either from other nerve cells or cells of certain other 

 parts of the organism. Until we know vastly more than 

 we do know of the chemical nature of intercellular substances 

 and of the chemical and physical activities which go on at 

 the planes of contact between cells, nothing could be more 

 gratuitous and unscientific than to assume that nerve cells 

 differentiate as they would were they not in some measure 

 vitally associated with one another from the very begin- 

 ning. Indeed, such positive knowledge as we have tends 

 strongly against such an assumption. 



Recall, for example, the fundamental part chemical mes- 

 sengers play in development. And Sherrington's insistence 

 on the role of intercellular substance and "surfaces of sepa- 

 ration" between cells in the functioning of the adult nervous 

 system is much to the point for this contention. Intercellu- 

 lar as well as intracellular conduction must be, he main- 

 tains, expected in the reflex-arc on the basis of the cell 

 theory.^ 



