Introduction 9 



bodies of the higher plants and animals have a 

 definite lower limit of size, being never smaller 

 than several (approximately five) microns in 

 diameter. ( 1 micron = 1/10,000 cm = 10,000 Ang- 

 strom units.) Since the diameter of the benzene 

 ring, as measured between the centers of the 

 carbon atoms, is about three Angstrom units 

 (3 X 10" 8 cm) in diameter, it would require a 

 series of about 17,000 benzene rings to extend 

 across the smallest nucleated cell. It is probably 

 safe to say that a structure of such complexity 

 could never have sprung into existence spon- 

 taneously from inorganic substances. The con- 

 clusion is therefore inevitable that the typical 

 nucleated cell does not represent the most primi- 

 tive form of life, but is probably the final result 

 of a long process of evolution. 



A much more primitive form of life is exhibited 

 by the bacteria which carry on the same processes 

 of metabolism and pass through the same cycles 

 of growth and cell division as their nucleated rela- 

 tives so that they must be regarded as true living 

 organisms. Bacteria differ from nucleated cells 

 in that they are definitely smaller in size, are 

 usually formed to resemble some simple geo- 

 metric figure, do not contain distinct chromo- 

 somes, centrosomes, or other self-perpetuating 



