THE FINAL ADJUSTMENTS 123 



a relatively minor influence of the external circum- 

 stances. 



It may be asked whether the organ-forming sub- 

 stances, which we mentioned when discussing mosaic 

 eggs, or the various gradients which we spoke of in the 

 eggs of newts and sea urchins, do not play at least as 

 important a part in determining development as the 

 genes themselves. If we think only of a single lifetime 

 they undoubtedly do. In a newt's egg, for instance, the 

 organiser is located at a particular place, related to the 

 gradient of yolk. The same is true of the various im- 

 portant regions we mentioned in insect eggs. But there 

 are two further points which we must remember. 

 Firstly, these various centres only become active when 

 a nucleus has passed into them, that is to say, when the 

 region of the egg can interact with genes in the nucleus 

 (see page 77). Secondly, we have to ask, how do these 

 regions of the egg become located in the right place? 

 The egg is a cell built up in the ovary of the mother's 

 body, where it undergoes, as we saw, a long period of 

 maturation and growth. Now during this period of 

 maturation the egg is influenced by the genes which 

 it contains: before the reduction division these are the 

 same as the genes in any other cell in the mother's 

 body. 



In organisms which have been thoroughly studied 

 we know many examples of genes whose most impor- 

 tant effect is to influence the process of egg matura- 

 tion; usually they lead to the formation of imperfect 

 eggs which fail to develop, so that they cause the female 



