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The growth and envelopment processes of three representative herpesviruses, equine abortion, pseudorabies, and herpes simplex, were examined in baby hamster kidney (BHK 21/13) cells by bioassay (plaque-forming units) and electron microscopy. The envelopment process was identical for all three viruses. After assembly in the nucleus, the nucleocapsid acquired an envelope by budding from the inner nuclear membrane. This membrane was reduplicated as the enveloped particle was released so that the budding process did not result in disruption of the continuity of the nuclear membrane. That portion of the nuclear membrane which comprised the viral envelope was appreciably thicker than the remainder of the membrane and exhibited numerous projections on its surface. Once enveloped, the viral particles were seen in vesicles and vacuoles in the cell cytoplasm. These appeared to open at the cytoplasmic membrane, releasing the virus from the cell. There was no detectable difference in the size or appearance of enveloped particles in intra- or extracellular locations.

The morphology and sequential development of infectious pancreatic necrosis (IPN) virus, a pathogen of trouts, were studied by electron microscopy. Mature virions were seen in the cytoplasm of infected cells incubated at 24 C as early as 6 hr after infection. These virions were hexagonal in profile and approximately 55 nm in diameter. Generally between 8 to 10 hr after infection, virus crystals of various sizes were occasionally observed. Although virus replication did not appear to be confined to a particular cytoplasmic locus, mature virions were sometimes seen in association with unidentified tubular structures approximately 45 nm in outside diameter. Negative stains of virus revealed unenveloped icosahedra approximately 65 nm in diameter with probably 92 capsomeres. Contrary to a previous communication which reported IPN virus to have picornavirus-like morphology, we found it to morphologically resemble members of the reovirus group.