C. H. WADDINGTON 



immediate gene products, or elements close to them in the reaction sequence, act as 

 gene-initiated plasmagenes, and have the capacity for identical self-duplication, we 

 should have to add to the diagram some further thick arrows similar to those which 

 symbolize the identical multiplication of the genes themselves. But until the empirical 

 facts force us to recognize the existence of such bodies, there seems no theoretical 

 need to postulate them. 



It is easy to see that in such cyclical systems, the switch mechanisms we have dis- 

 cussed, which serve to exaggerate the effects of small differences in initial conditions, 

 may be even more powerful. This would be so if the products of gene activity in- 

 creased the supplies or the suitability of the raw materials which that gene required. 



It follows from all this discussion that normal chemical kinetic theory provides 

 several different models for systems of interacting synthetic processes which 

 will change progressively towards one or other of a number of alternative steady 

 states, the decision between the various potential directions being made by the 

 initial conditions. These are just the characteristics which experimental analysis 

 leads us to attribute to developing cells. It is in terms of models of this kind that we 

 must envisage the fundamental processes of developmental physiology. The occur- 

 rence of gene-initiated plasmagenes with a limited autonomy is a possibility, but 

 whether they are actually of general occurrence must be left for future experiments 

 to decide. But in themselves they do not provide an adequate explanation either 

 of the progressive character of development, or of the occurrence of alternative end- 

 states. They should be regarded as possible elements in cyclical reaction systems of 

 the kind we have discussed rather than as providing an alternative model for the 

 understanding of development. 



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Ghromosomen struktur in der Entwicklung und Organdifferenzierung von 



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