REGULATION 155 



By what kind of nuclear and cytoplasmic changes is the specificity of 

 protein synthesis changed during development? No answer can be pro- 

 vided at present to this question. If one adopts the common — but un- 

 checked — assumption that each cell of a metazoan contains the complete 

 collection of mendelian genes, then the changes must concern the control 

 of protein synthesis. The difficult point is that these changes, although they 

 may not be genie, are transmitted clonally. One is left with guesses in- 

 pired by observations on simpler phenomena which share certain features 

 with differentiation, or on special cases of controlling mechanisms. 



The interplay of inducers and repressors of protein synthesis, for in- 

 stance, indicate a mechanismi by which the production of specific groups 

 of enzymes could be switched on or off. Not very complicated modalities 

 of such systems can indeed ensure the clonal maintenance of certain 

 metabolic states, and of specific protein synthesis (see p. 138). The known 

 cases of cytoplasmic mutations, in yeast, Eiiglena or Paramaecium, provide 

 a model for clonally transmitted restrictions of the capacity to make certain 

 specific proteins (see p. 141). For these reasons, the theories proposed for 

 explaining cytoplasmic heredity — whether they involved plasmagenes or 

 steady states — were often suggested as models of differentiation. The 

 wandering controlling elements found by McClintock (1956) to inhibit 

 certain genes in maize, or the episomic elements of bacteria (Jacob et al., 

 1960) also suggest mechanisms by which the mathematical rigidity of 

 mendelian control can be tempered. 



Processes of differentiation are often presented as being irreversible by 

 essence; or rather irreversibility is often taken as a criterion of differentia- 

 tion, i.e. as part of its definition. Although differentiation 'properly speak- 

 ing' may be irreversible, this irreversibility should not blind us, for it 

 may be merely a usual consequence of the type of process involved in 

 differentiation. Changes of cell constitution or cell structures which are not 

 irreversible, might be brought about by the same fundamental processes as 

 differentiation during development, but remain reversible because they are 

 less extent and less intricate. It might be very informative to know more 

 about the mechanisms of changes like sporulation, spore germination or 

 even the changes in enzyme constitution during the growth phase of 

 micro-organisms, changes in the mating type, i.e. in the constitution of the 

 cilia in Paramaecium; differentiation during growth of the slime mould 

 (Sussman, 1958; Wright and Anderson, 1958) might provide a transition 

 between micro-organisms and metazoans. It is true that in these simpler 

 processes differentiation is controlled or triggered by changes of the 

 medium, whereas in higher organisms interactions between cells (in- 

 ductions) play an essential part. But a few known facts indicate that the 

 outer agents of differentiation are not always very involved; simple 

 chemicals can have rather specific actions in some aspects at least of 



