12 HEREDITY AND ENVIRONMENT 



(b) Metabolism, or the transformation of 

 matter and energy within the living thing, in 

 the course of which some substances are oxi- 

 dized into waste products, with the liberation 

 of energy, while other substances are built up 

 into protoplasm, each part of every cell con- 

 verting food substances into its own particular 

 substance by the process of assimilation. 



(c) Reproduction, or the capacity of organ- 

 isms to give rise to new organisms, of cells to 

 give rise to other cells, and of parts of cells to 

 give rise to similar parts by the process of 

 division. 



(d) Irritability, or the capacity of receiving 

 and responding to impinging energies, or 

 stimuli, in a manner which is usually, but not 

 invariably, adaptive or useful. 



Both the egg and the sperm are living cells 

 with typical cell structures and functions, but 

 with none of the parts of the mature organism 

 into which they later develop. But although 

 they do not contain any of the differentiated 

 structures and functions of the developed or- 

 ganism, they differ from other cells in that 

 they are capable under suitable conditions of 



