30 



EVOLUTION AND GENETICS 



ment is called anabolism, the destructive part katabolism, and the 

 whole process metabolism ; this is one of the most striking charac- 

 teristics of living matter. Another distinctive quality is the 

 power to reproduce itself in the organized form which is charac- 

 teristic of the various species, and finally this remarkable sub- 

 stance has properties which enable it to receive stimuli of light, 



Centrosomes 



Golgl bodlea- 



PlaeiiQOBome or 

 nucleolus 



Chromatin 

 Llnln 



Karyosome 



True wall or membran9 

 Plasma-membrane 



Cortical layer 

 ' Plastids 



Mitochondria, eto« 

 Vacuole 



Metaplasm 



Fig. 12. — General diagram of a cell. (From Woodruff, after Wilson.) 



heat, contact, sound waves and chemical substances in its en- 

 vironment, to conduct these stimuli to various parts of the body 

 which it forms, and to respond to them in various ways with the 

 result that the organism fits into, or is adapted to its environment. 

 The Cell. Protoplasm as we see it in living organisms is found 

 in only one form, the cell (Fig. 12). No animal or plant exists 

 which is simpler than this unit, and all more complex forms are 

 built up of many such units. Cells may undergo great differentia- 

 tion of form as they become specialized for various tasks, and so 

 we find in the l^ody of man the flat, horny cells of the cuticle, tall 

 cells wiih waving cilia lining the trachea, long and contractile cells 



