248 ANIMAL AGGREGATIONS 



ing that the phase of the parent culture from which the inoculum 

 was taken affects the duration of this initial stationary phase. This 

 is the equivalent of the "lag phase" of Robertson (1924). 



2. The lag phase is also called the positive growth acceleration 

 phase, and is the period during which the rate of increase shows 

 acceleration. This period, together with the preceding one, makes 

 up the lag phase as so considered by many investigators working 

 with bacteria and with Protozoa. Buchanan and Fulmer use it as 

 meaning "the period elapsing between the beginning of multipli- 



15 20 



l//v/rs OF Time 



Fig. 21. — Showing the life-history of a bacterial culture. Figures on the vertical 

 axis give logarithms of numbers of bacteria; those on the horizontal axis give units of 

 time; those on the curve itself indicate the close of the respective growth phases. 

 (Redrawn, with slight modifications, from Buchanan and Fulmer 1928 by permission of 

 Williams & Wilkins.) 



cation and the beginning of the rate of maximum increase per or- 

 ganism." 



The phenomena associated with the lag phase interest us particu- 

 larly because of the inherent implication that an increase in numbers 

 or the continued occupancy of a new medium affect growth condi- 

 tions. A number of theories have been advanced in explanation, of 

 which the following may have significance in some cases (Buchanan 

 and Fulmer, 1928). 



a) The essential secretion theory is an application to the present 

 phenomenon of a variant of Semper's suggestion that some A^- 

 substance must be necessary for growth. Here it takes the form 

 that the transplanted organisms must give off some essential mate- 



