Vlll. MISCELLANEOUS FACTOKS 599 



by t'oodiii'i thjToid extract. In the future, metabolites essential tor the rat 

 and eertain other speeies may be best re\(>aled through the application of 

 external stresses to induce deficiencies. Similar stress devices have been 

 employed .succes.sfully upon microbial populations, through the use of anti- 

 metabolites, and the creation of nuitants that are incapable of carrying 

 out noiinal metabolic reactions. Pteroyl glutamic acid (Chapter 13) and p- 

 hydroxybenzoic acid (Chapter 12) were discoxered by these means. Since 

 the limit to the number of derived systems that ma^' be produced through 

 these approaches is remote, they offer many possibilities for discovering 

 pathways of metabolism that are at present obscured because of the ab- 

 sence of suitable test .systems. Turning again to "normal" strains of or- 

 ganisms, the fruitful researches on Tenebrio and other insects may signal a 

 shift of interest among investigators toward the lesser known phyla of the 



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