CHAPTER XVn 



THE SYNTHESIS OF NEW SUBSTANCES 



IN CONSIDERING Separately the influence of the cytoplasm on the genes, by 

 way of activation and inhibition, which was dealt with in the last chapter, 

 and the influence of the genes on the cytoplasm by the production of 

 substances active in development, we have made a distinction which, 

 however convenient, is to some extent artificial. It will be shown later 

 that in all probabihty the substances produced by genes at an early stage 

 of development are themselves capable of affecting the levels of gene 

 activity at later stages. The gene-cytoplasm complex is a single system, 

 between the parts of which reciprocal interactions occur. But it is easier 

 to discuss the individual steps in such circular reactions separately at first, 

 and to try to put them together again later. 



I. The parts of the cell 



Information about the production of substances by genes comes partly 

 from genetics, but very largely from cytological, embryological and bio- 

 chemical studies. It will therefore be advisable to begin this chapter by a 

 summary description of the structure of a typical embryonic cell, men- 

 tioning the cell-parts which are most important for the subsequent dis- 

 cussion (Fig. 17. i). 



The cell nucleus consists of the chromosomes, nucleoli and nuclear sap, 

 all contained in a nuclear membrane. In the chromosomes one can 

 roughly distinguish two types of material (see White 1954) : the euchro- 

 matin, which shows the typical staining behaviour from which the 

 chromosomes ('the coloured bodies') derive their name; and the hetero- 

 chromatin, of which there may be more than one kind, whose staining 

 behaviour diverges in various ways. The staining reactions of the euchro- 

 matin are in the main due to their content of desoxyribonucleic acid (often 

 abbreviated to DNA) which can be more or less specifically recognised 

 by the Feulgen reagent and less certainly by many other stains. This 

 substance becomes condensed on to the chromosomes at the time of cell 

 division, but it may spread more diffusely throughout the nucleus in inter- 

 phase. The different staining behaviour of heterochromatin is the result in 

 part of the fact that the phases of the nucleic acid cycle in it are not 

 synchronised with those of the euchromatin. It is probably also due in 

 part to a greater concentration in the former of the other type of nucleic 



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