36 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



cell, and occasionally a distinct rod-shaped body may be 

 observed. While many of the surface growths in the 

 culture experiments were pure cultures of cocci, the ma- 

 jority, perhaps, consisted of diplococci, clumped or agglu- 

 tinated cocci, and a type of coccus that I have called ''ring- 

 forms." In a few instances, rod-forms predominated in 

 the surface growths. Subcultures of these organisms fur- 

 nished a clue to the interpretation of this variation in mor- 

 phology. In some instances, where the initial growth con- 

 sisted of a pure culture of cocci, the subcultures were found 

 to contain all the varieties of forms mentioned above (see 

 plate III, facing p. 51). An extensive study of this pleo- 

 morphism was made, and it became evident that shght 

 variations in the culture medium were sufficient to bring 

 about remarkable variations in the morphology of the 

 microorganisms. These experiments will be considered 

 more fully in the next chapter. 



It was obvious that the results obtained so far did not 

 constitute conclusive evidence that mitochondria are bac- 

 terial in nature. There was, after all, no direct evidence 

 that the organisms grown in the culture media had origi- 

 nated from the liver cells. It was also possible that the 

 tissues used for planting might contain congenital con- 

 taminations or infections. This possibihty did not appear 

 likely, for, when the normal unincubated control tissue (a 

 part of the same liver used for planting) was minutely 

 examined there was no indication of any organism other 

 than the mitochondria. It again became necessary to seek 

 for some method that would conclusively demonstrate the 

 source of the variable organisms in the cultures. 



A new set of experiments was performed in which liver 

 tissue was planted in the same kind of media previously 

 used, but the tissue was permitted to incubate for only a 

 short time (from two to twenty-four hours in the different 

 tubes), when it was removed and treated with a mito- 



