8 Heredity and Environment 



a strong chemical affinity for certain dyes and hence is called 

 chromatin ; it also frequently contains one or more rounded 

 bodies which look like little nuclei and are called nucleoli. The 

 chromatin and nucleoli are imbedded in a substance which does 

 not stain readily with dyes and which is therefore called achro- 

 matin. Surrounding the nucleus is the substance of the cell-body 

 or cytoplasm and in this the various products of differentiation 

 such as muscle or nerve fibrils, secretion products and food sub- 

 stances are found. The cytoplasm often contains also a centro- 

 some which is a deeply-staining granule surrounded by radiating 

 lines and which is an organ for causing intra-cellular move- 

 ments, especially in connection with the division of the nucleus 

 and cell body. The nucleus and cytoplasm also contain more or 

 less water and inorganic salts, and all of these things taken to- 

 gether constitute what is known as protoplasm (Fig. i). 



Protoplasm is therefore organized, that is composed of many 

 parts all of which are integrated into a single system, the cell. 

 Higher animals and plants are composed of multitudes of cells, 

 differing more or less from one another, which are bound together 

 and integrated into a single organism. Living cells and organisms 

 are not static structures that are fixed and stable in character, 

 but they are systems that are undergoing continual change. 

 They are like the river, or the whirlpool, or the flame, which are 

 never at two consecutive moments composed of the same particles 

 but which nevertheless maintain a constant general appearance ; 

 in short they are complex systems in dynamic equilibrium. 



The principal physiological processes by which all living things 

 maintain this equilibrium are : 



(b) Metabolism, or the transformation of matter and energy 

 within the living thing in the course of which some substances are 

 oxidized into waste products, with the liberation of energy, while 

 other substances are built up into protoplasm, each part of every 

 cell converting food substances into its own particular substance 

 by the process of assimilation. 



(c) Reproduction, or the capacity of organisms to give rise to 



