GROWTH 537 



this with media of low nutrient concentrations (McPherson, Smith, and 

 Banta, 1932). 



Another possible interpretation depends on the presence or absence 

 of essential elements, both organic and inorganic, or on vitamin or 

 hormone-like effects. This field has hardly been touched, and investi- 

 gations here may clear up many problems concerning the nutritional 

 requirements and the responses of the organisms to various culture 

 fluids. The present tendency is to look in this direction for an under- 

 standing of variations in the reproductive rate, rather than to attribute 

 them to special allelocatalysts. Cf. Elliott, (1936), Hammond (1938), 

 Koser and Saunders (1938), Hall (1939), and other chapters of this 

 book. 



Another explanation of the effect of the volume of the culture on the 

 reproduction rate of the organisms might be that in larger volumes the 

 organisms use more energy swimming about, which would leave less 

 energy for reproduction. This view could be tested by the use of cine- 

 photomicrographic films in measuring the amount of activity of animals 

 in large and small isolation cultures, and correlating this figure with 

 the rate of multiplication. The relation might be different in rich and 

 in poor nutrient media and, if so, this would elucidate some of the 

 contradictory observations in the literature. 



Nutrition and Growth 



Protozoa (Ciliophora) feed naturally on bacteria, and with mixed 

 population of both it is difficult to analyze the growth. Maupas recog- 

 nized this difficulty in the nineteenth century and recommended that 

 Pasteur's methods be applied to the pure culturing of Protozoa. How- 

 ever, for some time little was done, other than to insure a uniform and 

 adequate supply of bacteria in the medium by cross culturing. 



Hargitt and Fray (1917) isolated and identified a number of bacteria 

 from protozoan cultures and endeavored to grow Paramecium on pure 

 cultures of bacteria, but found that no single species of bacteria was as 

 satisfactory food as mixed cultures. Bacillus subtilis was the nearest satis- 

 factory single species. Some species of bacteria were found to be toxic 

 to the paramecia, and other poisonous bacteria have been reported by 

 Kidder and Stuart (1938). Phillips (1922) extended the work of Har- 

 gitt and Fray and was unable to find a single species of bacteria suit- 



