A comparative light and electron microscope technique has been used to study the cytological changes accompanying teliospore (i.e. probasidium) germination in Ustilago hordei (Pers.) Lagerh. Special emphasis has been placed on determining the ultrastructural events involved in karyokinesls, especially meiosis, and cytokinesis. The thesis is divided into five parts, of which the first is concerned with pre-germinal differentiation. The great increase in microanatomical complexity which occurs during the pre-germinal stages is due largely to an increase in the amount of endoplasmic reticulum (ER) and to the formation of "primary hydration vacuoles." Evidently the nuclear envelope gives rise to the new ER which in turn dilates to form the vacuoles. This is accompanied by an increase in mitochondrial size and the development of patches of patches of "flocculent cytoplasm." Part II concerns the initiation and subsequent extension of the metabasidium (i.e. promycelium). Initiation involves the localized degradation of the inner spore wall, and deposition of new wall material. The ER and spherosome-like bodies seem to be associated with these activities. Once spore wall rupture has occurred the structural basis of promycelial extension is unknown but changes in the number, size, and distribution of the spherosome-like organelles appear to have profound effects on the differentiation of the organism. Septation, knee-joint formation, and budding are discussed in part III. Elaborate membrane complexes are associated with cross wall initiation. A membranous plate is completed across the cell before septal wall thickening begins. The initiation of sporidia (i.e. basidiospores) involves a localized plasti-cization of the promyoelial wall followed by degradation of the old wall and subsequent synthesis of new wall material. Bridge-formation results when two adjacent cells give rise to bud-like processes which grow together and subsequently fuse to produce a protoplasmic bridge. The structure and activities of the metabasidial nuclei and their associated structures are discussed in part IV, Both meiosis and mitosis are unusual in that the two chromatin bodies apparently remain attached to the centriolar-kinetochore-equivalent and at least one of the chromatin bodies in attached to the nucleolus throughout the division cycle. The results are compatible with Brown and Stack's (1971) model for somatic nuclear division in some fungi. Membrane complexes, resembling those which Initiate septa, form in association with prophase nuclei and maintain a specific relation with the nucleus throughout division. In part V the suggestion is made that these complexes form part of a mechanism controlling the positional relationships of nuclear and cell divisions in the promycelium. / Science, Faculty of / Botany, Department of / Graduate