CELLS OF BACILLUS MEGATHERIUM 67 



end of three hours, however, some of the cells showed evidences of 

 growth by an increase in size and a decrease in the number of gran- 

 ules contained in the cells, and at an increasingly more rapid rate 

 this process continued, the cells becoming longer and freer of gran- 

 ules; and after they did so they also divided. Three of the cells re- 

 mained throughout the period of observation without any signs of 

 growth, actually becoming smaller and more densely packed with 

 granules. 



It is evident from these micro-colonies that two kinds of growth 

 are occurring — an increase in the size of the cells, and an increase in 

 the number. A true measure of the rate of growth, then, is obtained 

 only by considering both, i.e., by obtaining the product of the number 

 and size of the cells. The relationship of these conditions to each 

 other is shown graphically in Figure 13, and the values from which 

 these graphs have been plotted are presented in Tables III, IV, and 

 V. In order to plot the three quantities, namely the number of cells, 

 the mean length of the cells, and the total cell length, on the same 

 scale, they have been converted to index numbers, the index in each 

 case being the mean value for the entire period of observation; that 

 is, the absolute number of cells has been converted into a percent; 

 age of the average number of cells, and the same process has been 

 carried through with the other tw^o values. In the first colony, where 

 the two cells both grew simultaneously, the curve for the average 

 length of the cells is interesting. It will be noted first of all that 

 there was a definite lag in this curve, active increase not being ap- 

 parent until after 180 minutes; that the cells showed no change in 

 size for a short period preceding each cell division; that the size 

 of the cells after cell division was greater than the size at the be- 

 ginning of observation, indicating that the increase in size is definitely 

 more than twice; and that the size attained after the first cell di- 

 vision was greater than the size just previous to the first cell division. 

 There is, then, a sort of rhythmical character to the curve for cell 

 size. The cells increase to a certain length, divide, then grow again to 

 reach a still higher length, divide, and so on. The increase in size 

 of course precedes the first cell division, so that if we plot the ab- 

 solute rate of growth in terms of total cell length, the lag is very 

 slightly shorter than the lag in the curve for numbers of cells, but 



