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A new class of filaments intermediate in diameter between actin and myosin filaments has been demonstrated in skeletal muscle cells cultured from chick embryos. These filaments, which account for the majority of free filaments, average 100 A in diameter. They may run for more than 2 in a single section and can be distinguished in size and appearance from the thick and thin filaments assembled into myofibrils. The 100-A filaments are seen scattered throughout the sarcoplasm at all stages of development and show no obvious association with the myofibrils. The 100-A filaments are particularly conspicuous in myotubes fragmented by the mitotic inhibitors, colchicine and Colcemid. In addition, filaments similar in size and appearance to those found in myotubes are present in fibroblasts, chondrocytes, and proliferating mononucleated myoblasts. The 100-A filaments are present in cells arrested in metaphase by mitotic inhibitors. Definitive thick (about 150 A) or thin (about 60 A) myofilaments are not found in skeletal myogenic cells arrested in metaphase. Myogenic cells arrested in metaphase do not bind fluorescein-labeled antibody directed against myosin or actin. For these reasons, it is concluded that not all "thin" filaments in myogenic cells are uniquely associated with myogenesis.

The parasitic protozoan Toxoplasma gondii has been examined with the electron microscope in order to study the fine structure and the formation of the membranes surrounding the cell . The study of the ultrastructure of the membranes covering the parasite shows the existence of a three-membraned complex . Only the outer membrane is considered to be the plasma membrane ; the two membranes below it form an inseparable whole of changeable molecular architecture (modifications in appearance depending on the methods of fixation, local differentiation) . During reproduction, which takes place by fission or more often by endogeny, the membranes of the daughter individuals are formed from the membranes of the parent . At first the middle and inner membranes of the parent extend, separating the cytoplasm of the daughter cells from that of the parent . The three-membrane complex of the endozoites is completed at the time of their liberation ; the external membrane of the parent covers the leaving endozoites ; thus, the plasma membrane of the daughter cells derives also from that of the parent . These findings on the origin and role of limiting membranes during reproduction differ entirely from those described so far for other cells .

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