SUMMARY: The ascus cytology of (Ces.) Rehm, a secondarily homothallic Pyrenomycete fungus, has been followed under the light microscope from the crozier, through karyogamy and the three nuclear divisions in the ascus (the first two constituting a meiosis and the third a mitosis) to the first spore mitosis and spore maturation. The spindles remain widely separated at division II, during which segregation for mating type is said to occur. After division III, in which the spindles are transverse, the ascus contains four pairs of sister nuclei. The realignment of the nuclei so that two non-sister nuclei are included in each of the four spores is brought about by the movements of the centrosomes or, more likely, of their outer portions. Spore delimitation, which starts from these organelles, appears as a thin cleavage line separating the spore plasm from the epiplasm of the ascus. A mitosis in the spore initial followed by degeneration of one of the four daughter-nuclei in the primary appendage leads to an imbalance in the mating-type factors in the spore. The wall of the mature spore is separable into three layers, the middle one of which is pigmented. The mucilaginous secondary appendage formed at each end of the spore is already present before the primary appendage is cut off.


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