During the growth cycle of Tetrahymena pyriformis the mitochondria undergo changes in position, number, and structure. Ciliates in the logarithmic growth phase possess elongated mitochondria which are aligned along the plasma membrane and are closely associated with the kinetosomes and kinetodesmata. Mitochondria appear to divide across the long axis at this time, resulting in two or more products. Throughout this phase of growth mitochondrial divisions keep pace with cytokinesis so that the population of mitochondria remains at essentially the minimal level. As the ciliates enter the stationary growth phase the mitochondria increase in number, become oval to spherical in shape, and some migrate into the cytoplasm. Intramitochondrial masses of various configurations appear at this time. Some of the mitochondria lying in the cytoplasm become incorporated into vacuoles. Within these vacuoles either a single mitochondrion appears or several mitochondria may be seen along with other cytoplasmic structures. Later in the stationary growth phase the contained mitochondria are dense and the tubules are more compact than normal. Various stages in disorganization of the mitochondria are observed in a single large vacuole. Cytochemical tests reveal the presence of acid phosphatase, suggesting that hydrolysis of the vacuolar contents occurs. Lipid droplets increase in number during the middle and late stationary phase of growth. These events are interpreted as being associated with the normal process of aging in T. pyriformis.

SYNOPSIS. The nephridial system of T. pyriformis consists of a contractile vacuole with its two pores and an elaborate network of tubules. Both the vacuole and its tubules constitute a continuous system of single membranes of approximately 70 Å in thickness. Tubular fibrils of about 250 Å extend from the pore wall to the vacuole membrane and appear to function in dilating the pore prior to systole. The nephridial tubules arise directly from the wall of the vacuole and are continuous with it throughout the vacuolar cycle. The tubules appear to be contiguous with the granular endoplasmic reticulum. A typical Golgi complex was not found. Some speculation about the interrelation of these structures as regards function is offered.

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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 81/~ hours which included the time required to induce synchrony by five heat shocks (41/~ 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 41/~ hours. Cytokinesis takes place at about 6 hours. The second division occurs about 21/~ hours later during which all the macronuclear events noted above are repeated.