THE DIFFERENTIATING SYSTEM 4II 



with two alternative types of differentiation (with and without active 

 tyrosinase and formation of melanin pigment), the choice between which 

 would be dependent on the previous history of the strain (whether it had 

 spent a period in a sulphur-deficient medium). 



There appears therefore to be no difficulty in accounting in one or 

 other of these ways for the existence of distinct alternative paths of 

 differentiation. Within a given path, there are two types of phenomenon 

 for which we have to provide an explanation. The first is the existence of 

 a range of variation of the kind which is exhibited in 'field' processes. 

 For instance, if some cells in the limb region enter on the path of develop- 

 ment leading to cartilage and bone formation, they may have the character 

 of the femur or, on the other hand, of some other part of the limb 

 skeleton. A somewhat similar phenomenon is that of 'modulation'. In this 

 case a given tissue assumes a range of histological forms in dependence on 

 the nature of its environment, but throughout all such changes retains its 

 essential character unimpaired. Presumably in both cases the variations are 

 basically quantitative in nature, and indicate that the concentrations of the 

 reacting gene-products and cytoplasmic substances can take a certain 

 range of values while still remaining within one and the same alternative 

 path of development. 



In contrast to such flexibility of behaviour is the essential permanence 

 which is alluded to by speaking of tissues as 'determined'. When tissues are 

 modulated by some external influence, they nevertheless retain their 

 original character and can re-express it when suitable conditions arise. 

 Can the hypothesis which attributes determination to competitive inter- 

 action provide a satisfactory explanation of this ? The question has been 

 little studied, either theoretically or practically. There is no doubt that 

 one can invent systems of competitive interaction which would, if they 

 actually existed, make differentiation very difficult to reverse. The mere 

 co-existence of numerous interacting substances would almost suffice. 

 Evolutionary changes hardly ever in practice get reversed, just because 

 they depend on such numerous gene changes that it is extremely improb- 

 able that all the reversals will happen simultaneously. In the same way, a 

 highly complex system of competitive interactions would scarcely ever 

 be brought to retrace its steps. But that is a very generalised argument; 

 and the occurrence of modulation, and of pecuHar combinations of 

 lability and fixity of character (such as the labihty of avian epidermis 

 which allows it to be induced to form a feather, combined with its fixity 

 of tract-specificity, p. 259), make one wish for some rather more detailed 

 understanding. This could probably come partly from a mathematical 

 investigation of the theory of stabihty of competitively interacting systems. 



