Figure 1. Phase-contrast light microscopic images of unstained cells of Bacillus megaterium. (a) Actively multiplying vegetative cells. (b) Sporangial cells containing phase-dark prespores. (c) Sporangial cells containing phase-bright immature spores. (d) Free mature spores after sporangial autolysis.

Figure 2. Electron microscopic image of a metal-stained mature spore of B. megaterium QMB1551 in transverse section. The spore contains a central core consisting of a faintly visible plasma membrane (IM) surrounding a cytoplasm which includes light-staining DNA regions (NP) and dark-staining ribosome granules (CP). Surrounding the core is a thin, darkly stained primordial cell wall and a contiguous unstained cortex (CX-PCW). Surrounding the cortex is a lightly stained inner coat, an intermittently visible outer membrane and a thin outer coat (C-OM). Enveloping all is a loosely fitting exosporium (EX). Bar, 200 nm.