500 FOOD REQUIREMENTS 



medium, in high concentrations, inhibits the growth of Chilomonas 

 Paramecium, while small amounts accelerate growth. Reich (1938) found 

 that the addition of culture filtrate produced no effect on the growth 

 of M. paleslinensis. However, Reich's filtrate was obtained from young 

 cultures (twenty-four-hour cultures in one experiment, for example) , and 

 his technique is not entirely comparable to that of Mast and Pace. Hall 

 and Loefer (1940), working with C. campyh/m in peptone medium, 

 found that the addition of old culture filtrates (one part in ten, to five 

 parts in ten) markedly increased the population yield, as compared 

 with that in control cultures. Furthermore, the growth rate, after the 

 first or second day of incubation, was noticeably higher in the cultures 

 containing old-culture filtrate. Acceleration of comparable magnitude 

 was also produced by the addition of aged sterile medium to fresh pep- 

 tone medium. In view of the latter observation, it now seems impos- 

 sible to attribute the effects of old-culture filtrates solely to a "biological 

 conditioning" of the medium, or entirely to a product or products 

 elaborated by the organisms growing in the medium. A basic explana- 

 tion for these various phenomena is not yet available, and it is possible 

 that any single explanation may be inadequate. Thus the "factor" of 

 Mast and Pace is said to be heat-labile, whereas the effects noted by 

 Hall and Loefer were produced by culture filtrates and aged sterile 

 medium which had been sterilized in the autoclave. At any rate, these 

 results are not only interesting in themselves, but they may also furnish 

 important clues in untangling the conflicting opinions concerning alle- 

 locatalytic and autocatalytic phenomena. For instance, some preliminary 

 observations of the writer have already shown that the "accelerating 

 factor" in old cultures may have a definite bearing on the growth of 

 C. campylum, in relation to initial density of population. 



Growth in Relation to Food Concentration 



It seems obvious that, within reasonable limits, the density of a 

 protozoan population should vary more or less directly with the con- 

 centration of available food until an optimal concentration is reached, 

 although the relationship might not be evident in the early history of 

 the culture. Such a generalization is supported by studies on pure cul- 

 tures. 



Cailleau (1933) noted that peptone concentrations of 3.0 percent sup- 

 ported abundant growth of Acanthamoeba castellan}}, while lower con- 



