A four-stage selection scheme is described for collecting and identifying temperature-sensitive mutants of a syncytiogenic strain of IHD vaccinia. Among about 6000 plaque isolates tested 90 were judged to be experimentally useful because of their adequate thermosensitivity and low rate of reversibility. Seven among the useful mutants proved to be defective in DNA synthesis. Most of the acceptable mutants were examined electron miscroscopically to ascertain the defects in morphogenesis which they manifest when allowed to replicate at the restrictive temperature. The wide spectrum of defects which became evident, each characteristic of a particular mutant, was schematized into an ascending order of 17 categories according to the complexity in development reached at 40°. The defects associated with each category are exemplified in a series of electron micrographs and accompanying descriptions.
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