CELLS OF BACILLUS MEGATHERIUM 71 



for the first sixteen hours, at which time growth had practically 

 ceased. It will be noticed that there was a lag period of about four 

 hours, after which time a maximum growth rate was quickly attained, 

 which continued for another hour, and then the growth rate began 

 to slow up, and there was a gradual decrease until the end of the 

 growth period. 



In this same graph the mean length of the cells is plotted. It will 

 be seen that this curve also presents a latent period, which is, how- 

 ever, shorter, ending at two and one-half hours. Then the 

 cells rapidly increase in length, and the increase is surprisingly 

 great, the maximum size reached at five and one-half hours being 

 approximately six times that of the original cells introduced into the 

 medium. Following this increase the cells as rapidly decrease again, 

 and at the end of nine hours they have practically reached the orig- 

 inal size. This period corresponds fairly closely with the point of 

 inflection in the growth curve between the period of active growth 

 and the resting period. From then on the size of the cells shows 

 slight fluctuations, but is relatively constant throughout the remain- 

 ing period of observation. x\lthough both curves show little fluctua- 

 tions due to experimental error, so that it is impossible to use the 

 data without smoothing for purposes of studying correlation, it is 

 quite evident from " the curves that there is a definite correlation 

 between the size of the cells and the rate of growth. The size of the 

 cells increases as long as the growth rate is increasing. It reaches 

 a maximum at a point corresponding to the point where the rate of 

 growth changes from positive to negative acceleration, and returns 

 to a minimum at a point corresponding with the point of inflection 

 between the phase of growth and the resting phase. 



It will be noted that while the curve for cell size shows a lag, 

 this lag period is shorter than the lag period in the curve for cell 

 numbers, and that, therefore, a change in cell size precedes actual 

 cell division, but that the lag period is not entirely occupied by this 

 change in cell size. Since the cells are increasing both in size and in 

 number, a true measure of growth is obtained only by plotting the 

 total cell length (assuming, as will be shown later is nearly the case, 

 that the cells do not change much in diameter, and that the length 

 of the cells is therefore a true measure of their size). Since in the 



