Electron micrographs of intracellular memngopneumonitis virus have shown several types of particles which are presumably representative of different stages of a life cycle. They are: (a) Elementary bodies—dense particles 250 to 300 mµ in diameter with very dense central granules. (b) Intermediate forms—less dense than elementary bodies and larger. They are 300 to 400 mµ in diameter, contain a very dense central granule, and often have two or three limiting zones, (c) Circles 400 to 500 mµ in diameter—homogeneous structures with single membranes and no internal granules. They are often elongated and constricted at the center in the manner of budding yeast cells, (d) Circles 500 to 600 mµ in diameter with single membranes.mdash;One, two, and three dense granules may be present in some of these structures, (e) "Larger" structures, circular to elliptical, often with discontinuities or ruptures in their membranes. They sometimes seem to have internal septa.

It has been inferred from these observations that the virus can multiply by binary fission or by multiple endosporulation and that elementary bodies are a spore-like stage.

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