CHAPTER XIX 



THE DIFFERENTIATING SYSTEM 



IT IS now time to try to formulate a general theory of differentiation based 

 on the various factors which have been discussed above. 



Perhaps the first problem which should be considered is the develop- 

 ment of differences between the various regions of the embryo. In the 

 scheme of intra-cellular reactions which was suggested earlier (see Fig. 

 1 6. 1, p. 349), the nature of the cytoplasm affects the course of events 

 at two different stages; on the one hand it provides the raw materials for 

 gene activity and may thus differentially activate or inhibit different 

 genes, and on the other it has the same relationship to the immediate gene 

 products (and plasmagenes, if any). It is therefore easy to see how the 

 constitution of the cytoplasm could set going a number of dissimilar 

 processes of differentiation. In fact, it would be quite possible for this to 

 occur through the interaction of the cytoplasm with the gene products, 

 even if the activity of the genes themselves was exactly the same in all 

 cells; but as we have seen there is actual evidence of nuclear differentiation 

 in the various tissues, and there seems no reason to doubt that both the 

 possible influences of the cytoplasm — on genes and on gene products — 

 are effectively in operation. 



The next point is the existence o£ distinct alternative pathways of 

 chemical change, leading to the production of a fmite number of defmite 

 tissues ; and the pecuhar mixture of permanence and lability revealed in 

 the phenomena of determination and modulation. The explanation for 

 the distinctness of the developmental paths can probably be found in the 

 nature of the cytoplasmic influences on the genes and gene products. All 

 the different genes are made out of similar building blocks, i.e. the amino- 

 acids and peptides which go to form the protein, and the nucleotides which 

 form the nucleic acids. The same situation holds for the gene products. 

 Thus we must suppose that the various genes (and gene products) will com- 

 pete with one another for the available raw materials. There may also be 

 other types of competition ; for instance a high concentration of one gene 

 product A may inhibit the formation of another B, and so on. There will 

 therefore be a situation of 'competitive interaction' in the formation of 

 the gene products and another in their production of cytoplasmic sub- 

 stances. 



406 



