250 ANIMAL AGGREGATIONS 



/) Penfold (1914) and later Robertson (1924) regarded favorably 

 the theory of elaboration of essential chemical substances. This theory 

 assumes that some substance, c, may be required for growth which 

 is not produced from a already in the solution, but from b which 

 must be produced by the organism from a. Further, c must be 

 present within the organism in optimal quantities before optimum 

 growth occurs; and in a medium lacking the proper amount of c 

 this substance cUffuses out of the bodies of the non-growing cells and 

 must be resynthesized before growth can take place. 



The causes of the lag phenomenon may reside both in the mechum 

 and in the organism. They may differ in different conditions, and 

 all the foregoing theories may operate in special cases. 



In the light of the situation concerning Robertson's phenomenon 

 among protozoa reviewed in chapter x, we are interested in the ef- 

 fects of number in the inoculum upon the duration of the lag period. 

 Robertson (1922) defined "lag" in terms of the initial stationary 

 phase and found no effect of numbers in the inoculum upon this 

 phase preceding the first division. His "allelocatalytic effect," in 

 which the rate of reproduction was much increased by the presence 

 of a second organism in a limited amount of culture medium, be- 

 longs in the lag phase as used here, and the entire discussion of chap- 

 ter X is pertinent at this point. 



3. The logarithmic growth phase (see Fig. 21 again) is the period 

 at which the rate of increase remains constant and at its highest 

 value. During this phase the logarithms of numbers of bacteria 

 plotted against time give a straight ascending line. 



4. Negative growth acceleration phase covers the time when the 

 rate of increase is falling, although the bacteria continue to increase 

 in numbers. This phase sets in soon in bacterial cultures but comes 

 more slowly in those of Protozoa. The causes contributing to its 

 onset and development are those which later lead to the decline in 

 numbers. These have already been discussed at some length in 

 chapter vi. Briefly, those considered most important are the in- 

 crease in concentration of harmful products of metabolism and the 

 decrease in available food supply, which lead to cells entering the 

 so-called "resting stage" or cause death. 



