DIFFERENTIAL GROWTH 163 



With that definition in mind we can now ask a number of specific 

 questions pertaining to the mode, the rate, and the site of growth. First, 

 do cells ever stop growing? Superficially, cells fall into two categories, 

 those that keep on proliferating, and those that do not. The ratio of the 

 former declines steadily during ontogeny. Yet even in the mature body 

 proliferation is still very active in many tissues, such as skin, blood- 

 forming organs, many glands, sex cords, and others. Other cell groups 

 become stationary. Since incremental growth can be observed only if 

 there is a net surplus of production over breakdown, it is quite con- 

 ceivable that the stationary cells are likewise engaged in growth, but at 

 a rate just sufficient to balance the metabolic losses. In this case, proto- 

 plasm would be continually regrown in all cells. 



What evidence is there for this view ? Our habit of lumping all syn- 

 thetic processes in one class and all degradation processes in another 

 tends to make incremental growth appear simply as a shift in the 

 balance toward the former ; in other words, simply as more of the same 

 kind of activity that is going on in any metabolizing cell. But if we 

 distinguish, as we must, between different levels and types of synthesis, 

 the problem loses its aspect of simplicity. To crystallize the issue let us 

 focus on the most complex molecular systems, the proteins. We know 

 from isotope tracer work (35) that proteins are in a state of continuous 

 renovation. Now, this may mean either that the individual molecule 

 merely interchanges constituent parts with its environment while pre- 

 serving its identity and individuality, or that the molecule breaks down 

 completely while another whole one, freshly delivered from the repro- 

 ductive system, appears in its stead. The difference is as between keeping 

 an automobile in repair by replacing the worn-out parts one by one, 

 or scrapping it and replacing it by a new one. If the latter interpre- 

 tation is correct, protein replacement and reproduction, i.e. true growth, 

 would definitely be two different processes. To the best of my knowledge, 

 the matter has not been finally settled. 



Let us then confine ourselves to the cells which are known to grow 

 perpetually (basal skin, lymph, intestine, etc.). Each of these cell types 

 is known to produce each more of its own kind. What does this imply 

 at the molecular level ? Again we are faced with alternatives that we can 

 only present, without deciding between them. The question is whether 

 the basic systems of reproduction are the same for all cell strains or 

 whether they change in character with progressive differentiation. 

 Perhaps all cell strains retain a certain germinal core population which 

 remains exempted from differentiation and constitutes the sole source 



