136 THE ACTION OF THE LIVING CELL 



peatedly confirmed, we may conclude that homologous 

 cytost afifects protozoans in a manner similar to that 

 found in tissue cultures: i.e., in very low concentra- 

 tion, it appears to exert a definite stimulation of ac- 

 tivity and growth, whereas in higher concentrations 

 it becomes distinctly toxic. The latter conclusion is 

 further substantiated by the observation that the ad- 

 dition of large quantities of homologous cytost to hay 

 infusion (a loopful per cubic centimeter) renders the 

 medium unsuitable for the cultivation of Paramecium. 



When we turn to the growth of bacteria, we find that 

 an enormous literature has accumulated concerning 

 the requirements for and the nature of bacterial 

 growth. Owing to the non-uniform nature of the 

 media employed in experiments concerned with such 

 studies, and to numerous conflicting statements as to 

 the results obtained, it is virtually impossible to draw 

 any definite conclusions concerning the general na- 

 ture of bacterial growth. 



Two undisputed facts, however, are worthy of com- 

 ment in connection with our discussion. It is now quite 

 generally accepted that, following the inoculation of 

 a bacterial culture, a definite time interval (the lag 

 phase) may elapse before active proliferation begins. 



Various hypotheses have been advanced to explain 

 this phenomenon : some believe that during the lag pe- 

 riod some substance essential for growth is liberated 

 into the medium from the cells of the inoculum ; others 

 have imagined that the lag phase represents a period 

 during which the organisms recover from injury 

 caused by the accumulation of toxic metabolites in 

 the parent culture. Penfold (1914) has suggested that 

 a time period is necessary for the synthesis of certain 



