72 THE CYTOLOGY AND LIFE-HISTORY OF BACTERIA 



pattern, unlike that of the reproductive filaments. Their function is dis- 

 tributive and they comprise the swarm. Evidence has been presented to 

 suggest that these sw^armer fdaments take part in a reproductive cycle, including 

 the formation of zygospores at the points of contact of swarms. This inter- 

 pretation, which is controversial, will be discussed in Chapters VI and VII. 



Other types of filamentous cell are common in bacterial cultures. Some 

 of these contain reduced or disorganised nuclear material. Their significance 

 is unknown, and they may be pathological. 



G: THE NATURE OF THE CHROMOSOME 



(9, 10, 38, 39, 42, 48, so, 51, 52, 54, 60, 61) 



It is dubious whether the bacterial chromosome exactly corresponds to 

 that of other cells. It performs the function of ensuring the correct distribution, 

 of genes, which may be assumed to be arranged upon it in a linear manner ; 

 and although in this particular it exactly resembles other chromosomes, it 

 differs from them in a number of respects. It is actually less susceptible than 

 the resting nucleus to radiations (Rubin, 1954), and being single, it does not 

 require an elaborate mitotic process to ensure correct distribution. 



Early genetic studies upon certain bacteria led to the adoption of theories 

 requiring the existence of three chromosomes or even of a branched chromo- 

 some ; but more recently it has been established that genetically, as well as 

 cytologically, a single chromosome must be assumed. 



In the process of fission it appears that each member of each pair of chromo- 

 somes is identical with the others, and is in fact derived from the same parent 

 chromosome in previous cell generations. This is quite unUke the condition 

 in plant or animal cells, where the two members of the pair may be of entirely 

 different constitution with respect to several genes. Similarly, when a smooth 

 bacterium divides, the chromosome pair at one end of the cell passes into 

 one daughter cell, and the pair at the other end, into the other daughter cell. 

 Thus the entire complement of four chromosomes is derived, at a remove 

 of two cell divisions, from a single, " grandparent " chromosome. Or at a 



