THE DIFFERENTIATING SYSTEM 



413 



say, which can produce crucial changes in the systems of competitive 

 interactions. We know that the initial evocating stimulus may be com- 

 paratively sHght, even a mere change in pH. We do not yet know on 

 exactly what part of the cell system this obtains its effect. Weiss (1947, 

 1949^) has pointed out that an evocating stimulus might act at the cell 

 surface, by causing the accumulation there of a particular molecular 

 species, with a consequent depletion of the deeper parts of the cell and an 

 alteration in the 'molecular ecology', or systems of competitive inter- 

 action (Fig. 19.3). There is, however, little convincing evidence that 

 evocating actions take place primarily at the cell surface (cf p. 213). 



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J 





Figure 19.3 



Diagram showing how two different substances (upper and lower rows) 



acting on the surface of a cell might attract to the surface different specific 



internal constituents, and thus cause progressive cellular differentiation. A 



conceivable mechanism for evocation. (From Weiss 19506.) 



Indeed the most critical evidence as to the location of evocator action 

 speaks in the opposite sense. Waddington and Goodhart (1949) in- 

 vestigated the position taken up within the cell by the sterol-hke hydro- 

 carbons which are extremely active evocators. It was found that they 

 are not absorbed on the surface or on the nucleus, but on lipo-protein 

 granules (lipochondria) in the cytoplasm, which then break down to give 



microsomes. 



