•GROWTH FACTORS 113 



values for the two organisms. The difference is due to a 

 factor which is active for Str. lactis but not for L. casei. 

 The factor has been isolated and it was found that 1/xg. of 

 it was equivalent in activity to 56 fig. of a folic acid 

 concentrate towards Str. lactis but that 1 /xg. was less 

 active than 0-004 jag. of the concentrate for L. casei. 

 This factor is different from the crystalline substance 

 described above which is more active towards L. casei 

 than towards Str. lactis. 



Folic acid, vitamin Be, the L. casei factor and xanthop- 

 NH— CO 



terin, nh=c C— N=C.0H , are closely related and 



I II I 



XH— C— N = CH 



probably concerned in the synthesis of thymine, 



NH— CO 



CO C.CH3 . Thymine can replace vitamin Be in the 



I II 

 ]N^H— CH 



nutrition of L. casei but not in that of Str. lactis R (identical 

 with Str. fcecalis). L. casei factor can be partially con- 

 verted into folic acid by incubation with chick liver ; 



Mycobacterium phiel Extract. — In 1912 Twort and 

 Ingram showed that Jo line's bacillus, on first isolation, 

 would only grow in the presence of some substance 

 occurring in an extract of M. tuberculosis honiinis or, 

 better, in M. phlei. The organism could be " trained," 



