Abstract
Summary and Conclusions
At first it was thought that 40 passages of the virus in chick tissue had increased its cytopathogenicity to the point where plaque production could occur. However ability of non-adapted strain to produce plaques refutes this conclusion. In retrospect it appears that no actual change in virus had occurred during serial passage, but the observation that CPE could be regularly produced after as long as 12 days led to use of above procedure and successful production of plaques appearing with this virus at relatively late time of 12 days after inoculation. The sudden appearance of plaques seems unusual; apparently the final change of CPE, resulting in loss of neutral red staining, must occur very suddenly with this system, resulting in overnight appearance of plaques.
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