124 HOW ANIMALS DEVELOP 



in question to be sterile. However, we know of some 

 genes which allow eggs to be produced, but affect them 

 in such a way that they always develop into individuals 

 with a peculiar characteristic. One of the most famous 

 examples is in a variety of water snail in which some 

 individuals are coiled in a left-handed screw and others 

 in a right-handed. This difference depends on the type 

 of egg which a given mother produces. That in its 

 turn is determined by the genes which she contains. 

 We see, then, that the arrangement of the various 

 regions of the egg, which plays such an important role 

 in development, is itself the result of the action of 

 genes in the previous generation. 



It is interesting to enquire what happens when an 

 organiser with one genetic constitution acts on com- 

 petent cells which have a different constitution. Ex- 

 periment shows that cells which react to an organiser 

 can only do so by expressing one or other of the 

 hereditary potentialities which they contain. It is, in 

 fact, their own genes which determine the range of 

 characters they can develop into. All an organiser can 

 achieve is to select one or other out of this range of 

 possibilities. ^ 



What is it that genes are doing that gives them such 

 overwhelming importance in development? To answer 

 this we need to decide first of all what actually hap- 

 pens in development. Now the study of living mate- 

 rials has led us more and more to the conclusion that 

 by far the most important constituents of cells are the 

 proteins. Nearly all the chemical reactions which go 



