60 MORPHOLOGIC VARIATION 



times the normal size, since in any given sample there will be just as 

 many cells recently divided as cells about to divide. As a matter 

 of fact, in their own observations, in a good many cases the maximum 

 size attained was considerably more than this amount, and according 

 to my own studies may be as much as six times that of the resting 

 cells. Moreover, if this is the explanation, it should apply equally 

 well to all bacteria, including the diphtheria group, which, as they 

 observed, decreased, rather than increased during the maximum growth 

 period. As will be seen later, there is much evidence that this change 

 in the size of the cells is correlated with other changes in their mor- 

 phological characters, and also in their physiology, and that the 

 real significance of the phenomenon is that the actively growing 

 cells are fundamentally different from the resting cells, and that 

 we have in a young culture a special morphological type which dif- 

 fers just as much from the standard type that we know from obser- 

 vations of twenty-four-hour cultures, as do the so-called involution 

 forms which we find in old cultures. 



I have studied this phenomenon of increase in size in some detail, 

 particularly with Bacillus megatherium, chosen because it is the lar- 

 gest bacterium that can be easily grown in artificial culture media, 

 such a large organism making measurements of size relatively easy 

 and more accurate than is the case with smaller bacteria. The strain 

 used was one isolated from soil, which corresponds with the published 

 descriptions of Bacillus megatherium in every respect save that it 

 is non-motile. This quality of motility in the megatherium group 

 seems to be one which is extremely variable. In addition to its 

 large size this organism is also characterized by the occurrence of 

 numerous intracellular granules, rather more refractile than the pro- 

 toplasm which contains them. They do not give the staining re- 

 actions of either volutin or fat; in fact they do not stain with any 

 of the dyes. But in the fact that they disappear during active 

 growth and reappear when growth slows up they behave like volutin 

 and possibly serve a similar function. 



The general character of the morphologic variations exhibited 

 by this organism may be seen in Plate 1 , which shows camera lucida 

 drawings of living cells removed from a culture at various stages of 

 growth. There was no appreciable variation in the cells in the first 



