A NOTE ON SPORE FORMATION 113 



seems to depend upon the density of the population in the culture. 

 He states that spore formation can be indefinitely postponed if the 

 culture is transferred to a new medium at a period preceding the 

 appearance of spores. It seems to begin after a definite number 

 of cell divisions have taken place, spore formation appearing at 

 a definite concentration of cells, regardless of the initial size of 

 the seeding for a given volume of medium, and begins always when 

 the medium is no longer suitable for vegetative multiplication, that 

 is when growth has ceased. Migula also found that spore formation 

 could not be delayed by the addition of nutrient to the medium in 

 concentrated form (dry peptone and meat extract), but, on the 

 other hand, by adding distilled water spore formation was postponed. 

 He concluded, therefore, that spore formation was not due to an 

 exhaustion of the medium; that it depended entirely upon the con- 

 centration of the cells, and was probably the result of an accumula- 

 tion of products of metabolism of the bacteria. 



In order to determine more clearly this relationship between 

 rate of growth and rate of spore formation, the following experiment 

 was performed: A culture of Bacillus cohaerens some days old, in 

 which there were practically no vegetative cells, was inoculated on 

 to four sets of agar slants, two of normal agar, the other two con- 

 taining one- fourth as much peptone and meat extract. One set 

 of each kind was inoculated with a very heavy suspension, and 

 the other with the same suspension diluted 1-50. At regular inter- 

 vals samples were removed and the percentage of free spores among 

 200 cells was determined from each sample. The results are given 

 in Table XXII, and are presented graphically in Figure 32. 



It will be noted from this graph that spore formation proceeded 

 more rapidly in the media of lower nutrient value than in the full 

 strength media, regardless of the size of seeding, and that it pro- 

 ceeded more rapidly in the heavily seeded cultures than in the lightly 

 seeded ones, though the difference here was not so pronounced as was 

 the effect produced by varying concentration of nutrients. In the 

 heavily seeded dilute medium, spores never entirely disappeared, 

 new spores starting to form before all of those introduced had germi- 

 nated. It would appear, therefore, that the rate of spore formation 

 is determined not by the concentration of cells alone, but rather 



