Physiology 473 



general ways: (1) that the process involves contraction of the vacuolar 

 membrane; and (2) that systole is produced by cytoplasmic pressure on 

 the vacuole. Although the change from a sol to a gel in the vacuolar 

 membrane might exert enough contraction to initiate systole (147), this 

 mechanism could not in itself bring about complete discharge. Cyto- 

 plasmic pressure theories maintain that contraction of the vacuole is 

 brought about by pressure of the cytoplasm against the vacuolar wall. 

 Observations on Amoeba dubia (214) and A. proteiis (382), in which the 

 vacuole becomes embedded in a zone of gelated cytoplasm just before 

 systole, suggest that pressure from this zone brings about systole. For such 

 ciliates as Paramecium, it appears that hydrostatic pressure is exerted by 

 a more or less fluid cytoplasm and that systole may be initiated by some 

 other factor, such as adjustment of the vacuole to the excretory pore. 

 Adjustment to the pore, as a preliminary step, would presumably insure 

 discharge of a full vacuole rather than a partially filled one. 



GROWTH OF PROTOZOA 



Individuals and populations 



The growth of individual Protozoa has been traced in very few 

 cases. In some species, a constant growth-rate has been reported, as in 

 Plasmodium praecox (193) and Ichthyophthirius multifiliis (373); in 

 others, a decreasing rate which may or may not follow a sigmoid curve. 

 Investigations on this phase of protozoan growth have been reviewed by 

 Richards (482). 



Cultures of Protozoa are essential for investigating a variety of prob- 

 lems. Pure cultures, containing one species of Protozoa and no other 

 microorganisms, are a necessity for tracing many biochemical and physio- 

 logical activities and for determining basal food requirements. "Species- 

 pure" cultures, containing one strain of Protozoa and one or more strains 

 of other microorganisms, also have been used to advantage in many in- 

 vestigations. The maintenance of such cultures with known bacteria in 

 known concentrations has insured reproducible experimental conditions. 

 In the case of Entamoeba histolytica, cultures of this type show promise 

 for preliminary investigations on growth requirements and amoebacidal 

 drugs (27, 477, 478). Mixed cultures, containing two species of Protozoa 

 either bacteria-free (37, 93, 182, 462) or with bacteria, also have been used 

 in a few investigations. An interesting example is the growth of Enta- 

 jnoeba histolytica with Trypanosoma cruzi (442, 443). Cause (140, 141, 

 142, 143) has been interested in the struggle for existence between com- 

 petitors for the same food supply (e.g., Paramecium caudatum and 

 Stylonychia mytilus feeding on bacteria) and in captor and prey relation- 

 ships (Didinium and Paramecium). In the competition between 5. mytilus 

 and P. caudatum, mutual inhibition was noted, S. mytilus being the 



