The fine structure of cells of Saccharomyces cerevisiae engaged in the formation of ascospores was studied in electron micrographs of ultrathin sections. Although the mode of the first reduction division could not be clearly determined, the second nuclear division appeared to proceed in a manner similar to that observed previously during vegetative division. That is, division by constriction of the existing nucleus occurs without dissolution of the nuclear membrane and without involvement of discrete chromosomes. Variously shaped areas of low electron density were discerned within the nucleoplasm; these had not been previously seen in the vegetative nucleus. The significance of this nuclear differentiation and its possible similarity to nuclear structures reported in bacteria and an imperfect fungus are discussed.

The cytoplasmic membrane appears first in the developing ascospore. The formation of an outer coat and an inner coat then follows. The cytoplasmic vacuole was observed not to be incorporated into the spore. An unusual intracytoplasmic membrane was observed in the spore and appeared to be at least temporarily continuous with the nuclear membrane.

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