THE ACTIVATION OF GENES BY THE CYTOPLASM 361 



by 'the condition of the cytoplasm' is still obscure; but, whatever it is, it 

 can be altered by a number of agents, for instance by the temperature at 

 which the strain is being cultured, the amount of food available, the osmo- 

 tic pressure of the medium, etc. This would seem to open the possibility 

 of discovering how the activation is brought about and the nature of the 

 cytoplasmic properties on which it depends. This is a clear-cut example 

 of the activation, by different types of cytoplasm, of different specifically 

 Corresponding genes; and the fact that this occurs, not in different parts 

 of a single body but in the various members of a strain of unicellular 

 organisms does not make the phenomenon any the less relevant to the 

 normal processes of development. 



3- Complete or partial inactivation of genes? 



There is therefore a fairly solid, and increasing, body of evidence which 

 indicates that the nuclei of differentiated tissues become influenced by 

 the cytoplasm with wliich they are associated. It remains to discuss how 

 far this influence extends. Are we to imagine that certain genes become 

 completely inactivated or even lost; or is it more likely that we are dealing 

 with a merely quantitative speeding up or slowing down of the gene- 

 actions? 



There is rather little direct evidence on the matter from the side of 

 embryology. It is true that in many plants almost any part of the organism 

 can be caused to produce a whole plant and must therefore contain the 

 whole set of genes. But plant tissues are not so higlily differentiated as 

 those of animals, and one can hardly adduce the evidence of their powers 

 of regeneration to prove that animal cells also always contain a full set 

 of genes. Among animals, it is difficult to fmd clear-cut cases in which 

 cells can be conclusively proved to have first differentiated in one way 

 and later to have shown a capacity to change into some other type which 

 might be supposed to demand the activity of previously unused genes. 

 When differentiated vertebrate cells are grown in tissue culture (Review : 

 Willmer i954), they 'modulate' into less-specialised forms which may 

 appear to be dedifferentiated, but they do not re-acquire the ability to de- 

 velop into some tissue other than the one from which they were originally 

 derived. However, there are fairly convincing cases of such a 'metaplasia' 

 (i.e. a renewal of developmental plasticity) in the ascidians, the embryos 

 of wliich are liighly 'mosaic' at a rather early stage while the adult cells 

 exhibit considerable flexibility during the processes of regeneration and 

 budding (Harrison 1933). We have also seen (Chapter XIV) that there is 

 good evidence for some degree of metaplasia during vertebrate regenera- 

 tion. 



