IX. METABOLISM 297 



that there might actually exist a more or less continuous spectrum 

 of size and chemical composition. Indeed, there are reports that the 

 microsomes and mitochondria may be separated into biochemically 

 different fractions. Novikoff et. al (130) have separated the cytoplas- 

 mic particles of rat liver homogenates into as many as eight fractions 

 by differential centrifugation, and have found evidence of biochemical 

 heterogeneity among the isolated particles. 



