164 PAUL WEISS 



of new protoplasmic compounds; in that case the synthesis of each 

 specific compound in skin, blood, gut, etc. would have to recapitulate 

 the whole course of the ontogenetic differentiation of that cell strain. 

 The new units would be turned out in a primordial mold common to all 

 cells and then gradually pass through a series of transformations after 

 the pattern of the already present skin-, blood-, gut-specific units, as the 

 case may be. The other possibility is that differentiation gradually 

 changes the reproductive apparatus of the various cell strains itself so 

 that the skin-, blood-, gut-specific systems would each be enabled to 

 reproduce themselves directly. If we were to postulate that the genes 

 represent the only self-reproducing systems, and assume, on the basis 

 of genetic evidence, that genes do not change in character during on- 

 togeny (60), the former alternative would become axiomatic. Some 

 new experimental contributions bearing on this problem will be reported 

 later. 



Growth Rate 



The fact that we do not know just what fraction of the molecular 

 population of a cell constitutes the actively reproductive growth appa- 

 ratus makes it impossible to determine "true" growth rates, i.e. the rate 

 at which the reproductive fraction procreates more of its kind^ This 

 places serious limitations on our interpretation of relative growth, for 

 in all our measurements of growth the reproductive and non-reproduc- 

 tive fractions of the cell are lumped. As long as their ratio remains 

 constant the error in estimating true growth from measurements of 

 total growth is constant and can be discounted for most purposes. For 

 example, if identical cells, kept in different conditions (temperature, 

 nutrients, etc.), grow at different rates, the difference can be properly 

 ascribed to effects on the true growth process. However, if cells of 

 different character grow unequally, even under identical external con- 

 ditions, this does not necessarily reflect differences of their true growth 

 rates; it may merely be an expression of the fact that different frac- 

 tions of their total mass are in reproductive activity. 



If we designate the reproductive fraction of the population as R and 

 the nonreproductive, hence, constant fraction as D, the true growth 



dR 



rate would be Vr = while the measured growth rate would be 



Rdt 



"^ True growth rate could theoretically be expressed by the time required for a unit 

 quantity of the reproductive system to increase to two units. 



