l68 PAUL WEISS 



neuron is strictly confined to the nucleated portion. It is probable, though 

 not proven, that the same would then hold for other cell types. 



These facts have an immediate bearing on efforts, currently in the 

 foreground of interest, to bring concepts of genetics and growth in line. 

 Our experiments place the site of growth near the nucleus, not neces- 

 sarily in the nucleus. We mentioned before that if the reproductive 

 sources should prove to be identical in all types of cells (as is assumed 

 for the chromosomal genes), we would have to assume that their prod- 

 ucts are instantly recast into specific patterns corresponding to the sur- 

 rounding differentiated cytoplasm. In our present case the nucleus may 

 continuously reproduce basic protoplasmic systems of the general 

 character dictated by the genie constitution, which, however, under the 

 molding influence of the existing specific neural population to which 

 they are added, would assume specific nerve-character. The alternative 

 would be that the intranuclear extrachromosomal systems themselves 

 have become specifically transformed in the course of differentiation 

 (see p. 144), and in that case the proliferated new compounds (nucleo- 

 proteins, etc.) could acquire their full nerve-specific character at their 

 intranuclear site of origin in statu nascendi. Our data are noncommittal 

 on this point. They do prove, however, that growth is not a ubiquitous 

 process in cytoplasm and should not be treated at par with the type of 

 synthesis commonly met with in other types of metabolism. 



Tissue Growth 



Analogous considerations, as applied in the foregoing to cell growth, 

 apply to the growth of tissues and organs. Just as the molecular popu- 

 lation of the cell consists of a reproductive and a nonreproductive frac- 

 tion, so the cellular population of a tissue is composed of proliferative 

 and nonproliferating components. In most organs proliferation becomes 

 gradually confined to certain restricted areas, so-called germinal zones, 

 arranged in layers (e.g. central nervous system, skin), cords (e.g. sex 

 cords) or foci (e.g. buds). Of the newly formed cells, some remain in 

 the germinal zone, while the rest move away from it. The former con- 

 tinue to proliferate, while the latter cease to multiply and usually un- 

 dergo some terminal specialization. 



This dichotomy has sometimes been interpreted as signifying an 

 antagonistic relation between proliferation and differentiation.^ Statis- 

 tically speaking, it is correct that as differentiation progresses, prolifera- 

 tion declines. But this does not imply that differentiation in the strict 



9 For a brief summary, see Weiss (47). P- 85. 



