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induction of different kinds of "petite" mitochondrial populations in Saccharomyces cerevisiae. J. Bacteriol. 90:481-494. 1965.-Mutant frequencies induced by 1 or 2 hr in 16 and 64 ,pg/ml of acrimutant forms. Chloroplast variants may be recognized by their pigments, rate of multiplication, plastid-cytoplasm interactions, and other traits (

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The mode of formation of conidia by a strain of Streplor~zyces viridochro~r~ogenes was followed by the observation of ultrathin sections of aerial hyphae a t various stages of sporulation. Before sporulation the aerial hyphae contain a long central strand of nuclear material. The laminated hyphal wall is covered with a l i b r o~~s non-spiny layer. A t equal intervals, and apparently simultaneously, the hyphal wall becomes involuted while the outer fibrous sheath becomes spiny. The nuclear material constricts, adjacent to the points of involution of the wall As the involution of the cell wall continues, only a thin isthmus of cytoplasm connects two adjacent fut~ire spores. The nuclear material breaks into spheroid struct~ires. The spiny fibrous layer brealrs into sporc size segments that enxelop each spore. r l n amorphous electron dense material accumulates between adjacent spores during maturation and eventirally participates in the formation of interspore pads. Spores in a chain are held together only by the fusion between adjacent interspore pads. As the spores in a chain becollie f~~l l y mature, an area of weakness can be seen between spore pads that are still held together. Although studied in less detail, the mechanism of spore formation is believed to be similar in a strain of S . violacezs.

SUMMARYUltrathin sections of sporangia of the actinomycete Microellobosporiu JEavea were examined with an electron microscope. The sporangial wall is a thin, wrinkled membrane which seems to be an extension of the outer layer of the cell wall of the sporangiophore. A substance, probably a liquid, is located between the sporangial wall and the sporangiospores. The spores are ovoid and consist of a laminated wall in which two layers can be differentiated. The wall is 30-40 mp thick but thickens to 90-120 mp a t the point of contact between two spores. Inside the spores one can differentiate a finely granular nucleus, large vacuoles which are probably filled with a fatty substance, and a coarsely granular cytoplasm. The cytology of the sporangiospores of M. Jlaveu is similar to that of the conidia of Walcsmaniu rosea.