The fine structure of the cell wall and the process of cell division were examined in thin sections of two unicellular blue-green algae grown under defined conditions. Unilateral invagination of the photosynthetic lamellae is the first sign of cell division in the rod-shaped organism, Anacystis nidulans. Symmetrical invagination of the cytoplasmic membrane and inner wall layers follows. One wall layer, which appears to be the mucopolymer layer, is then differentially synthesized to form the

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... um; the outer wall layers are not involved in septum formation. Centripetal splitting of the inner layer separates the two daughter cells. A second division, in a plane parallel to the first, usually occurs before the first daughter cells are separated. In the coccoid organism, Gleocapsa alpicola, the features of cell division are broadly similar; however, unilateral invagination of the lamellae is not observed and the second division takes place in a plane perpendicular to the plane of the previous division. = 0.26 mg (dry weight) per ml = 2.4 X 107 cells per ml. Under the same conditions, the generation time of A. nidulanis was 4 hr and an optical density at 750 nm of 0.2 = 0.16 mg/ml (dry weight) = 1.6 X 108 cells per ml. Electron microscope examinationt. Cells were harvested by centrifugation and fixed with Ryter-Kellenberger osmium tetroxide (21) or gluteraldehydeosmium under conditions described previously (2). 842 on May 7, 2020 by guest