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Abstract
An electron microscopic study of clamp formation and conjugate nuclear division was made of the somatic hyphae of Pleurotus ostreatus. Cytoplasmic organization of the growing clamp apex resembles that of the parent hyphal tip. One nucleus migrates into the clamp when the clamp has reached approximately half its mature size. A biglobular spindle pole body, associated with numerous microtubules, is present at the leading edge of the migrating nucleus. Separation of the globular elements occurs during migration. Mitosis of both nuclei is initiated when the clamp is fully formed. While the length of the spindle within the clamp is about half that of the spindle in the parent hypha, the paired mitoses are approximately synchronous. Septum formation delimiting the clamp cell precedes septum formation delimiting the progeny nuclei in the parent hypha. The clamp cell fuses with the subterminal hyphal segment. The subsequent migration of the progeny nucleus within the clamp cell into the subterminal hyphal segment reestablishes the dikaryon. Microtubules were not associated with the spindle pole body of this migrating nucleus.