DIFFERENTIAL GROWTH l8l 



amount of free active growth catalysts in a cell would thus vary with 

 the excess of one type over the other. Now, if we assume that the "posi- 

 tives" are retained in the cell (e.g. because of large dimensions), while 

 the "negatives" can escape, circulate, and enter other cells (e.g. because 

 of smaller size), we can see the general outlines of a highly specific and 

 sensitive mechanism of growth regulation, which would coordinate all 

 cells of common character into a single operational system, no matter 

 how much they are scattered. Local reduction (or augmentation) of 

 growth in any part of the system would imply a corresponding decline 

 (or increase) of both "positives" and "negatives" in that locality, and 

 because of their easier diffusibility, of the "negatives" only throughout 

 the system. Since systemic reduction of the "negatives" means an in- 

 creased ratio of active over conjugated "positives," a compensatory in- 

 crease of growth throughout the rest of the system will ensue; and con- 

 versely, enhancement of growth in one part will automatically entail 

 depression of growth in all other members of the system. Thus the total 

 growth of a given system would be held under joint control of all its 

 components. 



That some such mechanisms exist is becoming increasingly evident 

 from a study of compensatory growth reactions. That these mechanisms 

 operate, at least in part, after the fashion of immune reactions is indi- 

 cated by experiments in which organ-specific antibodies were shown to 

 have selective effects on the growth of the homologous organs (54). 

 But the precise mode of operation here sketched by way of illustration 

 is sheer supposition. Whatever its nature may eventually turn out to be, 

 one must remember that it constitutes merely another contributing fac- 

 tor to the highly intricate system of dependencies and interactions which 

 control the growth process. It is no more the master key to differential 

 growth than are any of the other factors discussed before. 



Conclusion 



What our whole discussion adds up to is that there is no single master 

 clue to the problems of differential growth or of growth in general. The 

 measurable growth of different parts is so intimately dependent on their 

 peculiar configurations and cellular differentiations, which themselves 

 differ sharply and qualitatively, that a purely quantitative comparison 

 of different growth processes is a deliberate abstraction. There are in- 

 stances where such abstraction has led to the recognition of striking 

 regularities capable of mathematical formulation (41) (e.g. allotropic 

 growth (19)). Whether these regularities are due to the fact that the 



