Abstract
Electron micrographs of ultra-thin sections of mature spores and of developing sporangia of Bacillus megaterium and B. cereus disclose the nature of the sporulation process. B. megaterium forms 2 spore coats, an inner and then an outer. B. cereus forms 3 spore coats, apparently simultaneously. The spore proper appears to be formed by a differentiation in situ of the sporangial cytoplasm. The spore coats appear to be formed by a condensation of the sporangial and spore cytoplasm. Various cytoplasmic inclusions are observed and discussed.