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SUMMARY: Spirillum lunatum has been shown to possess a life cycle in which there is an alternation between a vegetative and a microcyst stage. Microcyst formation may occur from the spiral vegetative cell by: (1) fusion of two entwined organisms to form one or more microcysts ; (2) the production of a protuberance a t some point along the organism into which the entire organism is gradually absorbed; (3) the gradual shortening and rounding of the organism to form an oval to spherical body. Microcyst formation begins, in a typical broth culture, a t c. 24 hr. after inoculation and the majority of the organisms are in the microcyst stage after a 4-day incubation period. When microcysts from an old culture are inoculated into fresh media, they germinate to form the spiral vegetative organism. Germination occurs by either unipolar or bipolar emergence of the germ tube.During a taxonomic study of the genus SpiriZZurn Ehrenberg (Williams, 1952), it was found that three species of the marine spirilla and two of the freshwater species exhibited morphological changes during the growth of the cultures. Young cultures, which consisted entirely of spiral organisms, gradually changed until only oval or spherical bodies (0*8-5*0p. in diameter, depending on the species) were found in old cultures. When these bodies were inoculated into fresh media, the normal spiral organism germinated from them. Since the entire organism is involved in the formation of these oval to spherical bodies, in a manner similar to the formation of the microcysts of the Sporocytophuga (Stanier, 1942; Grace, 1951), these bodies have been called ' microcysts '. Because of its relatively large size, S. Zunaturn was chosen as the experimental organism. The morphological changes which occur in this species are similar, although not identical, to those occurring in the other two marine spirilla and in the two freshwater species. METHODSThe spirillum was grown in sea-water nutrient broth of the following composition: peptone, 5.0 g.; beef extract, 3.0 g.; yeast autolysate, 3.0 g.; and sea water, 1000 ml. Broth tubes were inoculated with old cultures, ranging in age from 2 to 6 weeks, and incubated at 30". Organisms removed from the broth cultures at appropriate times between 8 and 72 hr. were used for microcultures. Cultures less than 24 hr. old were centrifuged and the harvested organisms re-suspended in a small amount of sterile sea water. Cultures older than 24 hr.

Migula, in 1900, separated the genus Spirillum Ehrenberg from the other spiral bacteria on the basis of motility by means of tufts of polar flagella. Mligula mentioned the fact that the tuft, in certain species, was often agglutinated

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