Virions: Mature virions are typically embedded within the proteinaceous viral inclusion body structure. The dark-staining core is cuboidal when cut in one plane, while biconcave when cut in the other. The core is 50 nm wide in the broadest part of the biconcave cross-section, and narrows to 5-10 nm in the center; it is approximately 200 nm long. Surrounded by a lighter-staining shell, the core variesfrom a thickness of 35 nm to 60 nm. A darker-staining area, 25 nm thick, forms a further external shell. the virion's overall dimensions are approximately 300nm x 260 nm x 140 nm.