80 MORPHOLOGIC VARIATION 



elusions can be drawn as from the micro-colonies, though perhaps 

 the evidence is more clear, namely that the period of maximum 

 growth is accompanied by an increase in size of the cells, and that 

 this is correlated with the rate of growth, being greater with lightly 

 seeded cultures than with more heavily seeded ones. The maximum 

 size corresponds with the moment of maximum growth rate. There 

 is a lag period in the curve for cell size, and the lag period in the 

 curve for cell numbers is therefore not entirely occupied by the 

 transformation of the small cells to large, growing ones. But the 

 cells do increase in size before they begin to divide, and if we con- 

 sider growth both from the standpoint of cell size and cell numbers, 

 that is the total growth in the mass of proptoplasm, then the 

 lag phase is correspondingly shortened. 



Not all of the inoculated cells grow at the same rate; first a few 

 and then more and more cells commence to grow, manifesting their 

 growth by increase in size; and by studying the frequency distribu- 

 tion of the cells according to their size, we can demonstrate this 

 variation in growth potentiality of the cells inoculated into the 

 medium. This degree of variation is greater with smaller seedings, 

 with very small seedings there being a pronounced tendency for bimo- 

 dality to develop in the distribution curves, indicating that under such 

 circumstances there does occur, as was postulated by Tenfold, a selec- 

 tion of a rapidly growing strain from the inoculum. That this de- 

 pends upon the size of the seeding will be apparent after a moment's 

 thought. If we inoculate a culture with two cells, there are two pos- 

 sibilities: First, that both of these will grow at the same rate; second, 

 that they will grow at different rates. Because of the tendency to 

 variation inherent in living organisms, the chances are in favor of the 

 second possibility. If we have three cells, then the chances are 

 that there will be three different growth rates, and by steadily in- 

 creasing the number of cells we increase the possibility of having 

 a perfectly graded series of cells with regard to their growth rate 

 when transferred to a new medium. In a culture thus inoculated 

 with a large number of cells those cells which have the maximum 

 growth potentiality will of course start to grow soonest, but they 

 will be followed quickly by the cells having the next highest growth 

 rate, and so on; and we will thus have a perfectly graded series 



