THE ACTIVATION OF GENES BY THE CYTOPLASM 



349 



the cytoplasm, or with other substances which are already there. There is 

 likely to be quite a complicated series of reactions of this sort before the 

 appearance of the differentiated cytoplasmic constituents which charac- 

 terise the various fully developed tissues of the adult animal. As these 

 reactions go on, the cytoplasmic environment of the nucleus will be 

 altered, and this wdl affect the nature of the nucleus-cytoplasmic re- 

 actions. The gradually changing constitution of the cytoplasm may also 

 be expected to alter the nature or intensity of the activities of the im- 

 mediate gene products. We must therefore be prepared to fmd ourselves 

 faced with a double cyclic system, of the kind pictured in Fig. 16.1. 



IMMEDIATE 

 GENE PRODUCTS 



CYTOPLASMIC 

 PROTEINS 



Figure 16.1 



The double cycle of intra-cellular reactions. The genes in the nucleus acting 

 on cytoplasmic substrates, both reproduce themselves and control the form- 

 ation of 'immediate gene products', a, b, c, etc. These then use cytoplasmic 

 raw materials (i) perhaps to reproduce themselves identically, in the mamier 

 of plasmagenes, and (ii) to elaborate the final cytoplasmic constituents, 

 P, Q, R etc., which are the substrates or raw materials which condition 

 the activity of both the genes and the gene products. (From Waddington 



I954fl.) 



(4) The whole system is organised in such a way that development 

 tends to proceed along one or other of a number of alternative canalised 

 paths. That is to say, the developmental reactions tend to go towards one 

 defmite type of end-result or another; and while development is going 

 on, the system has some power of self-regulation, so that no efiect is 



