PART Ill.-GENERAL PRINCIPLES 



THE CELL AND THE INDIVIDUAL 



670. Spontaneous Generation. — It was at one time held that 

 some animals originate spontaneously. In the middle of the 

 seventeenth century the great anatomist, Harvey, expressed 

 the view that all living things spring from eggs (Omne vivum 

 ex ovo). But this opinion was not generally accepted. A 

 quarter of a century later another anatomist, Redi, showed that 

 the maggots which develop in decaying flesh are bred from the 

 eggs deposited by flies. But for two centuries more spon- 

 taneous generation was thought to account for the appearance 

 of many living things, though it came gradually to be limited 

 to the microscopic organisms, like the bacteria and protozoa. 

 Finally, in the latter half of the nineteenth century the experi- 

 ments of Pasteur and others definitely established the view that 

 even for these microscopic forms a Hving germ is necessary to 

 development of a living organism. It was shown that if the 

 germs of the organisms which produce fermentation and decay 

 were carefully excluded from the substances in which they 

 usually occur, that the processes of fermentation and decay 

 would not take place and the associated organisms would not 

 appear. 



671. Continuity of the Living Substance. — At the present 

 time the term egg is applied only to special cells produced by 

 multicellular organisms, from which new individuals develop, 

 but the unicellular organisms produce spores which are, in 

 this sense, the counterpart of eggs. Another objection to 

 Harvey's phrase may be made on the ground that new individ- 

 uals may be produced by methods, such as budding, fission, 



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