The macronuclei of synchronously dividing mass cultures of Tetrahymena pyriformis (strain WH6) were examined with the electron microscope for changes during two division cycles. Samples were prepared at 30-minute intervals for a period of 8½ hours which included the time required to induce synchrony by five heat shocks (4½ hours). The interphase macronucleus contains peripheral, crescent-shaped nucleoli and evenly distributed chromatin bodies. Centrally located RNA bodies, composed of fibers, appear 1 to 2 hours following the initial heat shock. They are completely destroyed with ribonuclease whereas the nucleoli are only partially so. Following the third heat shock the RNA bodies move to the periphery and disintegrate; the nucleoli aggregate and form blebs which protrude into the cytoplasm where they appear to pinch off and may contribute to the cytoplasmic ribonucleic acid. Cytokinesis does not occur at this time. Instead the nuclear events are repeated during the 4th and 5th hours, even though the heat shocks are terminated at 4½ hours. Cytokinesis takes place at about 6 hours. The second division occurs about 2½ hours later during which all the macronuclear events noted above are repeated.

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