SUMMARY: A technique is described for fixing and embedding dormant ascospores of , whose impermeability has made them difficult subjects for electron microscopy. The ultrastructure of dormant and germinating spores has been compared. Endoplasmic reticulum occurs relatively infrequently in dormant ascospores whereas germinating ones have large amounts. An unusual structure, consisting of concentric membranes of varying degrees of complexity, appears after activation and disappears after extension of the germ tube. This membrane complex is continuous with the endoplasmic reticulum and may be its origin during germination. Mitochondria in dormant ascospores are larger and less numerous than in germinating ones. Two new elements of the ascospore wall are described, one of which may contribute to the relative impermeability of these forms. Nuclei increase in number during germination, but the exact number of divisions is not known.


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