Stages in the development of perithecia of Neurospora crassa, designated by the time elapsed after crossing, were investigated with the scanning electron microscope, from protoperithecia through perithecia. The usual examination of external features of whole specimens with this instrument was augmented by a freeze-fracture technique which allowed the viewing of development intemally as well. Rapid increases in perithecial size soon after crossing were followed by the appearance, in section, of

more »

... centrum, at first undifferentiated but subsequently developing ascogenous hyphae. The perithecial beak appeared as a compact mass easily distinguishable in whole specimens from the surrounding hyphae by means of texture as well as shape. Two ascospores were photographed during emergence from an ostiole, but ostioles were found more frequently closed than open.