90 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



made (Chapter IV), and which occurs in many different types of bacteria, 

 in one of a number of similar forms. Fusion cells appear, containing three 

 pairs of chromosome complexes, in the case of smooth cubacteria, and similar 

 fusion nuclei in other types of bacteria. The fusion cell grows into a long 

 filament, undergoing two nuclear divisions, and eventually fragments into 

 individual bacteria, usually in multiples of three. 



As this process includes an appearance of nuclear fusion and reorganisation 

 it may be regarded as sexual, vegetative reproduction. There is not enough 

 evidence to suggest whether the initial fusion, which produces the type of 

 cell containin^T diree pairs of chromosome complexes fused at its centre, is 

 autogamous or truly sexual, but the process is obviously quite different from 

 simple, asexual fission. It is, however, a vegetative process, directed towards 

 the purpose of increasing the number of bacteria in the culture, and not, as 

 in the case of the later sexual fusions, concerned with the distribution and 

 perpetuation of the species. As a method of reproduction it probably is not 

 much less efficient than simple fission, because it does not hinder growth 

 by any serious reduction in the proportion of cell surface to volume, as the 

 formation of a more typical symplasm might do. Some energy must be 

 required for the initial fusion, but this need not necessarily be a great loss, 

 and the disadvantage is presumably outweighed by the advantage of the 

 redistribution of nuclear material. 



As the behaviour of the chromosome complexes during simple fission 

 is such as to suggest that a rapid segregation of heterozygotes would probably 

 occur, it is possible that sexual, vegetative reproduction may assist in the 

 redistribution of genctical characters in the culture, prior to the change of 

 nuclear phase which usually follows. 



Sexual vegetative reproduction may be found at any stage of culture, 

 but is most common in cultures aged from twelve to forty-eight hours, and 

 may often supersede simple fission almost entirely. Cultures in this condition 

 may develop a macroscopically rough appearance, because of the high 

 proportion of filamentous cells ; but this is transient, and disappears as the 

 culture matures. 



In myxobacteria, filamentous cells bearing this type of nuclear structure 

 move actively with the rest of the swarm, and exhibit sufficient co-ordination 



