CONTROL OF CULTURES 473 



interesting work which has been done regarding accessory growth fac- 

 tors and nutritional supplements. A large number of these are consid- 

 ered in the chapter on pure cultures (Chapter IX). It might be sug- 

 gested, however, that one of the most fertile fields of protozoan investi- 

 gation has been opened up with the development of bacteria-free tech- 

 niques, and our knowledge of unsuspected requirements in the nutrition 

 of carnivores should be extended greatly in the near future. The possi- 

 bilities are many along these lines, and therefore considerable time has 

 been devoted to the methods which will have to be employed in the be- 

 ginning of any such studies. 



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