SUMMARY. Chromosome behavior of strains WH‐6, and WH‐14 mating types I and II respectively of variety 1, of T. pyriformis was studied during conjugation. Meiosis takes place during the first two prezygotic divisions. The chromosomes are at late pachytene‐early diplotene during the maximum elongation of the crescent in prophase of the first micronuclear division. As the crescent shortens, five bivalent chromosomes are readily recognized. Five dyads go to each pole at the end of the first division. Following a brief interphase the second meiotic division results in four micronuclear products, each with five chromosomes. Three of these products disintegrate; the fourth undergoes mitosis resulting in two pronuclei each with five chromosomes. Following a reciprocal interchange of the migratory pronuclei between the two conjugants, the synkaryon of each is formed just anterior to the macronucleus. The diploid number of ten chromosomes has been counted at the first division of the synkaryon.

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&njugating animals of the protozoan, Tetrahymena pryiformis, were affixed to cover slips by means of Nissenbaum's fluid, followed immediately by 1 3 acetic-alcohol for 18-24 hr. After fixation, the material was transferred through a descending alcohol series to water, then hydrolyzed in 1 N HCI, washed in water, followed by immersion in 45y0 acetic acid and subsequent mounting in aceto-carmine. Photomicrographs were made using a phasecontrast microscope and Microfile film. The schedule resulted in preparations with abundant material, adequate spacing of chrornosomes in a single plane, and excellent differentiation of the chromosomes from the cytoplasm.

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