PART TWO 



Genetics and Development 



Genetic factors are identified in the first instance by the analysis of 

 the inheritance of characters. Now that the mechanism of inheritance 

 is known, in its main outlines at least, it is possible to tackle the next 

 question, of how the genes affect the developmental processes which 

 convert the fertilized egg into the adult organism. Genetics here links 

 up with experimental embryology as another aspect of the general 

 biological problem of reproduction. The first chapter of this part 

 gives a summary of modern views on the causal processes of develop- 

 ment in animals, with especial reference to what is known, from the 

 experimental embryological side, of the role of the nucleus. The next 

 rv\^o chapters discuss the actions and interactions of genes which control 

 the kinds and quantities of substances produced during development. 

 A separate chapter is devoted to the effect of genes on animal patterns, 

 which is connected with the fundamental problem of the development 

 of biological form. The last chapter deals with the special problem of 

 sex; there are few topics of general biological importance to which 

 genetics has made more significant contributions. 



