FOOD REQUIREMENTS 491 



than in an ammonium-nitrate medium, the two media differing only 

 with respect to the nitrogen source. Hence asparagin may actually be a 

 poor nitrogen source for Euglenidae, even in light, and any growth 

 stimulant might produce an effect comparable to that noted by Dusi in 

 E. pisdformis. It should be noted, also, that the serial-transfer technique, 

 in which Dusi apparently used two-drop inocula, might require an 

 increase of as much as a hundred times in each transfer, if the original 

 density of population is to be maintained. In the writer's experience 

 with E. anabaena, the increase in asparagin medium was never greater 

 than twenty-five times in any tranfer, and was often less. Hence the 

 possibility exists that Dusi's rate of dilution in serial transfers was much 

 more rapid than the growth of his flagellates in media without growth 

 factors, and that the use of larger inocula might reveal E. pisciformis to 

 be capable of slow growth in asparagin media. 



If growth factors are actually essential, all the chlorophyll-bearing 

 Phytomonadida which have been investigated appear to synthesize such 

 substances from the constituents of suitable inorganic media. The color- 

 less species, P. uvella and P. obtusum (Lwoff and Dusi, 1938a), show 

 the same synthetic ability in salt solutions to which acetate has been 

 added. Polytoma ocellatum and P. caudattim (Lwoff and Dusi, 1937b, 

 1937c) apparently require thiazole for growth in such media, while 

 Polytomella caeca (A. Lwoff and Dusi, 1937a, 1938a, 1938b, 1938c) re- 

 quires both thiazole and pyrimidine. A. Lwoff and Dusi (1937a) have 

 shown that P. caeca grows fairly well in an asparagin medium, and much 

 more rapidly after the addition of thiamine or of thiazole and pyrimidine. 

 They have assumed, accordingly, that the growth in asparagin alone was 

 dependent upon a trace of thiamine in the asparagin itself. The same 

 interpretation is also applied to several other flagellates, on the basis 

 of similar evidence. Just as in the case of C. Paramecium, several dif- 

 ferent pyrimidines and thiazoles accelerate the growth of P. caeca (Lwoff 

 and Dusi, 1938b, 1938c), and several thiazoles are also effective with 

 P. ocellatum. 



Among the Zodmastigophora, Sir/gomonas oncopelti (M. Lwoff, 

 1937), 5". culicidarum, and S. fasciculata (M. Lwoff, 1938b) appear to 

 require thiamine, which cannot be replaced by thiazole and pyrimidine. 

 The last two species require hematin in addition to thiamine. 



Of the Sarcodina, Acanthamoeba castellami (A. Lwoff, 1938b) ap- 



