tion, never the reverse. This seems to indicate to the author an 

 evolutionary trend in bacteria from polar flagellation to peritrichous 

 flagellation. Typical strains of the Aeromonas genus show pre- 

 dominantly polar monotrichous flagellation in cultures which have 

 attained a relatively dense population. In very young or very Hght 

 cultures several strains showed peritrichous flagellation in addition 

 to the polar. The lateral flagella usually have a shorter wave- 

 length than the polar flagellum. Although the polar flagellum may 

 be of the normal or undulant type the lateral flagella are alike. One 

 widely distributed strain, indistinguishable from Aeromonas phys- 

 iologically, has coiled peritrichous flagella and appears to be a 

 stable mutant. A well authenticated mutation has been observed 

 in a strain of Lophomonos. Lophomonas has polar multitrichous 

 flagella of long wavelength like the spirilla. In one culture typical 

 of the genus, lateral flagella of relatively short wavelength were 

 observed in addition to the typical polar flagella. Individuals with 

 only peritrichous flagella were also observed. The latter were 

 isolated in pure culture and have remained unchanged over a 

 period of years. This culture is indistinguishable from typical 

 Alcaligenes species with curly flagella. In several strains of Rhizo- 

 biiim with a single subpolar flagellum, occasional individuals with 

 one or more subpolar or lateral flagella of very short wavelength 

 have been observed. Most often the normal subpolar flagellum 

 and the curly flagella are found in the same individual, but occa- 

 sionally organisms are seen with only the curly flagella. Pure 

 variants of Rhizobiwn with only the curly flagella have not been 

 isolated. 



Variation in Motility 



The change in motility associated with change of flagellar shape 

 has been discussed. Yet to be mentioned is the complete absence 

 of motility observed in some strains with otherwise normal flagella. 

 This phenomenon has been observed in Salmonella and Listeria. 

 These "paralyzed" mutants have shown fair stability. 



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