Morphological studies of orf and vaccinia viruses.

Abstract Electron microscopy of cytoplasmic extracts of cells infected with orf and vaccinia viruses has, with phosphotungstate contrast, shown two main kinds of virus particle of each. The apparently complete form consists of a nucleoid with surrounding envelope material, and the incomplete form consists of envelope material with or without nucleoid components. The nucleoid of orf is a round-ended cylinder with a distinct pattern produced by threads which are thought to represent nucleoprotein. Suggested arrangements of this are discussed. The nucleoid of vaccinia is closely similar, has a less clearly defined pattern, and appears to be a more highly organized structure, as does its incomplete form.