l62 PAUL WEISS 



pepsin, etc., belong in the latter class and hence can divulge nothing about 

 the reproductive systems, which are still very much of a mystery. At any 

 rate the distinction here proposed enucleates the core of the growth 

 problem, as it were. We shall say of a pigment cell that it grows not 

 when it synthesizes more melanin granules, but only when it synthesizes 

 more of that peculiar part of its protoplasm which, other conditions 

 favorable, can synthesize pigments. Similarly, growth of a myoblast is 

 not the synthesis of more myosin and actin, but of more of the basic 

 muscle protoplasm possessing, among others, the faculty to synthesize 

 myosin and actin ; and so forth. For the sake of simplicity let us ignore 

 for the moment the fact that "more" refers not to absolute production, 

 but to the net excess of production over destruction in opposite metabolic 

 phases. It would also be premature to regard the "basic" systems in 

 question as nothing but enzyme mixtures, although enzymatic activity 

 is one of their prime characteristics. 



The fundamental feature of growth, therefore, is the capacity of a 

 reproductive system to procreate more systems with similar reproduc- 

 tive faculty. In comparison with this fundamental phenomenon of 

 multiplication of the molecular key population, the often superimposed 

 phenomenon of cell division is as incidental as the setting up of new 

 administrative districts in a growing country. The relation between 

 cell division and growth is by no means as close as the widespread habit 

 of treating them interchangeably would make it appear. During cleav- 

 age, an egg undergoes repeated cell divisions with practically no growth 

 at all, while conversely, certain cells, such as mature neurons or the 

 single-celled kidneys of some nematodes, grow to comparatively enor- 

 mous sizes without division of either cytoplasm or nucleus. 



The growth process is being explored from many directions and with 

 increased momentum. Pertinent information comes now from genetics, 

 now from virus research, now from embryology. Views begin to con- 

 verge, and concepts, to become unified. Yet the more diversified the 

 contributions, the more we must guard against the obvious danger of 

 confusing a common label with a common object. If growth means cell 

 division to one, increase in weight to another, elongation to a third, 

 metabolism to a fourth, protein synthesis to a fifth, their mutual com- 

 munication on "growth" is apt to become deceptive unless they are fully 

 explicit. I want to make it clear, therefore, that I shall use the term 

 strictly as defined above, that is, as the multiplication of that part of 

 the molecular population capable of further continued reproduction, 

 irrespective of whether or not accompanied by cell division. 



