572 LORANDE LOSS WOODRUFF 



which gives the rate of division of each of the hnes in the dif- 

 ferent volumes of medium averaged for each four days of the 

 experiment. 



This series of experiments covered seventy-two davs and was 

 run simultaneously and coextensively with those already outlined 

 with which it is compared, and all the conditions to which each 

 was subjected were identical, the only factor requiring special 

 mention being the food content of the culture medium. It is 

 stated in the description of the cultures of the Ad series of experi- 

 ments that the culture medium was made up on alternate days — 

 so that the organisms in this series were continued for two days 

 on infusion that was made up at the same time, but which was 

 supplied fresh (from the stock flask) at the end of the first 

 twenty-four hours. Thus the only difference in the treatment 

 of series Aa and Ad was that the change to a second supply of the 

 culture medium was not made at the end of twenty-four hours. 

 Therefore the only difference in the environment of series A a and 

 Ad, at the beginning of the second twenty-four hours, was that 

 Ad was put m a fresh portion of the medium in which it had been 

 for the past day, and consequently a medium not contaminated 

 by its own excretion products, whereas Aa was continued for the 

 next twenty-four hours in the same portion of medium in which it 

 had been living for the previous day. It is beheved that cross 

 infection rendered the bacterial flora of the infusion in the supply 

 flask and that on the Aa culture slides essentially the same and 

 observation also showed that the supply of bacteria on the cul- 

 ture slides was ample. 



The results, then^ of this series of cultures in which the organisms 

 were isolated every forty-eight hours, confirms the general result de- 

 rived from the series isolated every twenty-four hours, i.e., that an 

 increased volume of medium is conducive to more rapid muUipdca- 

 tion,^^ and further it clearly shows that the gain in rate of division 



29 Attention is called to the fact that Aa-20 gained 0.05 per cent more over Aa-2 

 than did Aa-40 and therefore is an exception. Cf. fig. 4, B. It is possible that 

 another factor enters, or at least becomes perceptible after twenty-four hours in a 

 volume as large as forty drops. The bacteria in this quantity of culture fluid may 



