CELLS OF BACILLUS MEGATHERIUM 85 



by increased variation in size as well as by increased size of the 

 cells. 



It will be remembered that Chesney demonstrated that the rate 

 of growth is also dependent upon the age of the cultures used for 

 inoculation, and this factor has also been studied with regard to 

 the size of the cells. A series of agar slant tubes prepared as de- 

 scribed in Chapter III were inoculated wdth a culture of Bacillus 

 megatherium; samples were removed every hour and measures of 

 cell length made. After tw^o, four, six, and eight hours, subcultures 

 were made onto other agar slant tubes prepared in the same way. 

 An attempt was made to keep the size of the seeding constant 

 by preparing suspensions of as nearly as possible the same degree 

 of turbidity, as determined by inspection. The results are presented 

 in Figure 23, which shows the mean length of the cells plotted 

 against the age of the culture. It will be seen that the parent culture 

 showed in general the same type of curve as has previously been 

 described. There was a slight latent period, after which the cells 

 increased in size, reached a maximum, decreased rapidly and then 

 more slowly until they reached the original size. In the culture 

 transplanted from the parent culture when it w'as tW'O hours old, 

 the cells continued to increase in size at the same rate as in the 

 parent culture, but reached a somewhat higher maximum and main- 

 tained their large size for a somewhat longer period of time. In 

 the culture transplanted from the parent culture when it was four 

 hours old, the cells showed a somewhat further increase in size, 

 but at a somewhat lower rate, and the increase in size of the four- 

 hour subculture over the two-hour subculture was less than the 

 increase of the two-hour subculture over the parent culture. By 

 continually subculturing during the maximum growth rate, then, 

 we can continue to increase the size of the cells, but this increase 

 is not proportional, the amount of increase being smaller with each 

 succeeding subculture, so that there is a definite limit to the size 

 attainable. In the culture transplanted from the parent culture at 

 the end of six hours, the cells immediately started to increase again, 

 and reached a somewhat higher maximum than in the parent culture; 

 but when transplanted at the end of eight hours, that is at a time 

 when the cells in the parent culture had returned to their minimum 



