98 HEREDITY AND ENVIRONMENT 



mate unit of living matter which is capable of 

 independent existence. Neither the nucleus 

 nor the cytoplasm can for long live independ- 

 ently of each other, but the entire cell can 

 perform all the fundamental vital processes. 

 It transforms food into its own living material, 

 it grows and divides, it is capable of respond- 

 ing to many kinds of stimuli. But while the 

 parts of a cell are not capable of independent 

 existence they may perform certain of these 

 vital processes. 



Not only is the cell as a whole capable of 

 assimilation, growth and division, but every 

 living part of the cell has this power. The 

 nucleus builds foreign substances into its own 

 substance, and after it has grown to a certain 

 size it divides into two ; the cytoplasm does the 

 same, and this process of assimilation, growth 

 and division occurs in many parts of the nu- 

 cleus and cytoplasm, such as the chromosomes, 

 chromomeres, centrosomes, etc. In all cases 

 cells come from cells, nuclei from nuclei, 

 chromosomes from chromosomes, centrosomes 

 from centrosomes, etc. 



Indeed, the manner in which all living mat- 



