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Electron microscope observations of 2-celled embryos from 8 strains of laboratory mice revealed an abundance of intracisternally located type-A particles. A much smaller number of the same type particle was seen in blastocysts from the same strains. Blastocysts and egg cylinders from highly leukemo-genic AKR mice also contained budding and extracellularly located type-C particles. Unfertilized eggs of all strains investigatedl acked any visible viral particles.
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Biczysko et al. (1973) studied this question.
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