MORPHOLOGY OF BACTERIA 27 



indentation in the cell wall which increases in extent until 

 the division is complete and there exist two individuals. 



Ordinarily these new or daughter cells completely sepa- 

 rate from one another after division, but occasionally they 

 remain together for a shorter or longer time in the form of 

 pairs, chains or clumps. Under certain conditions of nutri- 

 tion long threads or filaments are formed. When these are 

 cultivated under more favorable life conditions they break 

 up into short fragments. When the culture media are ex- 

 hausted or nearly so, the rod-shaped forms often divide with- 

 out the usual elongation, resulting in a mass of coccus-like 

 forms ; but when these are brought under favorable life con- 

 ditions they assume their original shape. It is now thought 

 that in the reproduction of bacteria the division starts from 

 the protoplasmic layer, the central portion being passively 

 divided. This has been used as an argument against the 

 true nucleus. The time required for the complete process, 

 of division in most species of bacteria under favorable con- 

 ditions is from 15 to 30 minutes. According to investiga- 

 tions by Buchner 1 spirilla double in number under, most 

 favorable conditions in 20 minutes. 



Although elongation in the greater diameter and trans- 

 verse division is the rule for most bacteria, there are certain 

 groups, such as the sarcina, which divide more or less regu- 

 larly in three planes and instead of becoming separated 

 from each other there is a tendency for the segmentation to 

 be incomplete, and the organisms to remain together in 

 masses. In other groups, such as the micrococci, the divi- 

 sion takes place in two planes. When these are held to- 

 gether we have tetrads. The rod-shaped bacteria have not 

 been observed to divide longitudinally. 



Longevity of bacteria. The duration of life of indi- 

 vidual bacteria is theoretically unlimited, since from each 

 organism by the process of fission two new ones are pro- 

 duced. The life of these organisms, therefore, has, because 



Buchner, Centralb. f. Bakt., Bd. II (1887) p. 1. 



