FOOD REQUIREMENTS 499 



These results are interpreted as supporting Robertson's concept of allelo- 

 catalysis, although Reich does not subscribe to the theory of an auto- 

 catalyst of growth. 



In observations on Etiglena, Hall and Schoenborn (1939b) have noted 

 that the population tends to reach a concentration which is more or less 

 independent of the initial density of population under the conditions 

 described, and thus that the increase in the population varies inversely 

 with the initial density of population. These conclusions were based 

 upon counts made after specified periods of incubation, and population 

 curves were not traced. 



At present it is impossible to correlate the results which have been 

 obtained with bacteria-free cultures, although the cited observations all 

 indicate that the initial density of population influences the rate of 

 population growth. Since the experiments on various species have been 

 carried out under different conditions and in different media, it is pos- 

 sible that some of the puzzling contradictions may eventually be traced 

 to differences in technique, rather than differences in the nature of popu- 

 lation growth. In fact, different relationships between the density of 

 population and the rate of growth might reasonably be expected under 

 different experimental conditions. 



Growth in Relation to Waste Products 



Investigations on so-called waste products of Protozoa have led to con- 

 flicting opinions. Woodruff (1911, 1913) concluded that waste prod- 

 ucts inhibit growth of the homologous species, although growth of a 

 different species may be relatively unaffected by the same substances. 

 The other extreme is represented by Dimitrowa's (1932) observation 

 that the growth of Parameciutn caudatum is accelerated by the addition 

 of small amounts of old medium to the experimental cultures. More 

 recently, W. H. Johnson and Hardin (1938) have observed no significant 

 effects of old culture medium on the growth of P. multimkronucleata. 



Very little work on this problem has been carried out with pure 

 cultures. The observations of A. Lwoff and Roukhelman (1929), that 

 growth of G. piriformis ceases long before the food supply is exhausted, 

 has lent some support to the view that growth may be inhibited by 

 accumulated waste products. The results of later investigations are not 

 so readily interpreted. Mast and Pace (1938) have noted that old culture 



