The following mural and nuclear events were scored by light and electron microscopy from induction, the initial event, to meiosis I, the start of sporulation: flocculation, copulation, conjugation-tube formation, cross-wall formation, cross-wall erosion, conjugation-tube expansion, cytoplasmic fusion, de-differentiation of site of union, nuclear migration and karyogamy. The following were identified as new structural elements: sex hairs, which presumably mediate H bonding between cells during flocculation; a crimp at the site of union; a dark patch, which presumably serves as a leak-proof seal at the time of cross-wall erosion; a suture or electron-dense seam formed by the union of a copulant pair; and small electron-dense particles close to the site of wall erosion.