The cell physiology of early development 



of simultaneous differential equations, each containing terms of the second order 

 such as p 2 or pq etc. He shows that such a system may be expected to exhibit a number 

 of alternative steady states, some at least of which are likely to be stable, and that 

 the particular one which the system actually attains will in many cases depend on the 

 initial conditions. 



This sort of investigation seems to me to provide the bare skeleton of the theoretical 

 outlook which we require to understand the mechanisms of differentiation. The 

 field would, I think, repay much more study than it has yet received. In particular, 

 one would like to see a further discussion in terms of open systems rather than closed 

 ones. I hope that some competent mathematician, at home in the field of chemical 

 kinetics, will interest himself in it. 



GENES 



IMMEDIATE 

 GENE PRODUCTS 



f'PLASMAGENES"?! 



CYTOPLASMIC 

 PROTEINS 



Figure 2. The double cycle of processes in a differentiating cell. 



I should like to pass on to the last point I want to mention about the physiology of 

 the developing cell. The genes both draw the raw materials they use to make their 

 gene-products from the cytoplasm, and pass the products themselves back into the 

 cytoplasm. Similarly the raw materials for the synthetic processes going on in 

 the cytoplasm (controlled by the immediate gene products, either with or without the 

 intervention of plasmagene-like factors) are taken from the cytoplasm while the 

 synthesized products are returned to it. We must therefore expect to have to deal 

 with cyclical processes. In fact, there must be two such cycles, one by which the 

 cytoplasm controls the activity of the genes while the latter change the character of 

 the cytoplasm; and a second similar cycle involving the immediate gene products 

 rather than the genes themselves. The system of the developing cell can therefore 

 hardly be more simple than that represented by the diagram Figure 2. If some of the 



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