The fine structure of Bacillus subtilis has been studied by observing sections fixed in KMnO4, OsO4, or a combination of both. The majority of examinations were made in samples fixed in 2.0 per cent KMnO4 in tap water. Samples were embedded in butyl methacrylate for sectioning.

In general, KMnO4 fixation appeared to provide much better definition of the boundaries of various structures than did OsO4. With either type of fixation, however, the surface structure of the cell appeared to consist of two components: cell wall and cytoplasmic membrane. Each of these, in turn, was observed to have a double aspect. The cell wall appeared to be composed of an outer part, broad and light, and an inner part, thin and dense. The cytoplasmic membrane appeared (at times, under KMnO4 fixation) as two thin lines.

In cells fixed first with OsO4 solution, and then refixed with a mixture of KMnO4 and OsO4 solutions, the features revealed were more or less a mixture of those revealed by each fixation alone.

A homogeneous, smooth structure, lacking a vacuole-like space, was identified as the nuclear structure in a form relatively free of artifacts.

Two unidentified structures were observed in the cytoplasm when B. subtilis was fixed with KMnO4. One a tortuous, fine filamentous element associated with a narrow light space, was often found near the ends of cells, or attached to one end of the pre-spore. The other showed a special inner structure somewhat similar to cristae mitochondriales.

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