DIFFERENTIAL GROWTH l6l 



Another point is that our concept is perhaps too exclusive in assign- 

 ing the major ordering role to the surfaces. Some faculty of self-sorting 

 of molecular mixtures (as in the formation of tactoids [3] ) will perhaps 

 have to be brought into the picture to make it complete. Also our assump- 

 tion that selective transmission effects and affinities are essentially 

 explicable in terms of steric conformance of specifically shaped mole- 

 cules remains to be verified. It is still possible that ultimately we may 

 have to take recourse to some other sort of "resonance" mechanism, 

 "resonance-like" interrelations among tissues being clearly recognizable 

 on the biological level (46), 



Growth 



The problem of growth likewise resolves itself now into a number 

 of tangible issues. The definition of growth gains precision. Growth is 

 often defined as any increase in the mass of an organic system. Since 

 such an increase may occur by the mere intussusception of water — and 

 even a dead seed can swell — this popular definition is scientifically of no 

 value. I propose to define growth in a more restricted sense, as the 

 increase in that part of the molecular population of an organic system 

 which is synthesized within that system. The other part of the popula- 

 tion, consisting of inorganic substances like water and salts, passing in 

 and out of the system, is left out of consideration, and so are the smaller 

 organic compounds that enter as ready-made building blocks, e.g. sugars 

 and amino acids, until they have become assimilated and thereby lost 

 their identity. Since, as far as we know, the synthesis of the complex 

 high molecular systems in a cell occurs only in the presence and with the 

 intervention of other similarly complex and cell-generated systems, after 

 their own image, growth in our definition is essentially synonymous 

 with reproduction. In population terminology, we might say that im- 

 migration of molecules into the cell space as such does not constitute 

 growth, but their assimilation into complex systems after the model of 

 preexisting indigenous systems does. 



We can thus divide the cell content roughly into two categories : 

 systems capable of turning out more of their own kind, and others 

 devoid of that faculty — reproductive and non-reproductive ones. The 

 non-reproductive group includes all the elementary ingredients for the 

 higher compounds, as well as the sterile terminal cell products — mem- 

 branes, fibers, secretions, pigments, etc. In thinking about these matters 

 it is rather distressing to note that the proteins about which we have the 

 most direct information, such as collagen, keratin, melanin, myosin, 



