384 



Annals New York Academy of Sciences 



partially purified material which is not absorbed on anion or cation exchange 

 resins; the material may be precipitated with 67 per cent ethanol in the cold, 

 contains carbohydrate, protein, and no nucleic acid or lipids. Attempts to 

 replace this fraction with known substances, thus far, have been unsuccessful. 

 However, it has been possible to demonstrate a purine and pyrimidine require- 

 ment for the organisms, as well as their need for a number of vitamins, in a 

 medium (table 1) supplemented with this factor. 



Purine requirements for particle containing and particle free animals are 

 summarized in table 3. The need for exogenous source of a purine derivative 



THE EFFECT OF PENICILLIN ON LAMBDA 

 POPULATION 



None 



lUvml 



7 



Figure 2. 



is apparent and may be met by guanosine and guanylic acid. The free base, 

 and adenosine and its derivatives, do not replace the purine. Apparently, 

 Paramecium converts guanosine to adenosine and its derivatives, whereas 

 the reverse reactions do not occur. Inosine, its derivatives, and xanthosine 

 and its derivatives failed to replace guanosine as a growth requirement. 



The pyrimidine requirements may be satisfied by uridine, cytidine, uridyhc 

 and cytidylic acids (table 4). The free bases uracil, cytosine, and thymine, 

 as well as thymidine and thymidylic acid were not effective in replacing uridine 

 or cytidine. These data confirm earlier work with P. miillimicronucleala, 

 P. caudahim, and other strains of P. aurelia. 



By means of C^'^-labeled purines, it has been shown that adenosine is in- 

 corporated into nucleic acid adenine only, whereas exogenously supplied guano- 



