60 INTRODUCTION TO CYTOLOGY 



it contains something necessary to the performance of that activity, 

 much as a chemical stockroom "controls" the activities carried on in the 

 laboratory. 



In a mature organism metabolic activity maintains the individual in 

 the living condition. In a young and developing one it also results in 

 growth and the differentiation of structural and functional characters. 

 The manner in which the various constituents of the protoplast partici- 

 pate in metabolism and hence affect the course of development is studied 

 by observing the effects of altering this or that constituent. In the case 

 of the nucleus it has been found that differences in its constitution are 

 associated with particularly clear-cut differences in certain of the char- 

 acters developed. This leads to the subject of the role of the nucleus 

 in development and heredity, which is to occupy our attention in later 

 chapters of the book. 



