Sequential stages in the development and release of ECHO 9 virus have been illustrated and described. It is suggested that viral particles differentiate and become oriented in columns upon a fine filamentous lattice at cytoplasmic template sites which are distinct from the endoplasmic reticulum. Subsequently, virus is dispersed in the peripheral cytoplasm and gains egress from the cell through rents in the plasma membrane. Complete cellular disruption with viral release may supervene. The virus consists of a 13 to 15 mµ dense core and a poorly defined outer membrane, 22 to 24 mµ in diameter. Incomplete forms, lacking the core, are observed in the cytoplasm but have not been seen in the extracellular space.

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