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Ultrastructural features of micronuclear meiosis occurring during conjugation of the ciliate P. aurelia are described. Following pair formation the micronuclei enter prophase. The core of condensed chromatin in the micronucleus fragments, and almost the whole nucleus becomes filled with coiled and twisted chromatin elements. After about 1 5 hr of pairing regions of condensed chromatin appear linked into strands 1-2 p;m long. Somewhat later, ill-defined, synaptinemal, complex-like material lies between paired masses of chromatin. Only a few synaptinemal complex-like structures can be seen in the nucleus. A short while later, synaptinemal complex-like material may be seen free in the nucleoplasm. After about 25 hr of pairing, the micronucleus expands into the crescent stage. Microtubules appear in the nucleus, which expands rapidly to a length of about 20 /Lm, usually oriented along the long axis of the cell. Concurrently with expansion the chromatin becomes diffuse and spreads along the length of the nucleus. This stage probably corresponds to diplotene. The nuoleus then shrinks, miorotubules reappear, and metaphase of the first meiotic division oocurs. A large number of chromosomes, with indistinct kinetochores, can be seen. At telophase the nucleus is 15-20 /Lm long and dumbbell-shaped. It breaks into two parts, there being no separation spindle, in contrast to mioronuclear mitosis. The seoond meiotic division occurs immediately. Ultrastructurally it resembles the first with respeot to metaphase, anaphase, and telophase. Since P. aurelia has two micronuclei, there are eight haploid meiotic products. The one nearest the paroral cone survives and the other seven disintegrate and are resorbed. The remaining nucleus undergoes a modified mitotic division. One of the two final produots becomes a stationary ("female") pronucleus, while the other becomes a migratory ("male") pronuoleus. Male pronuclei are exchanged aoross the fused paroral oone regions of the partners. The male pronucleus contains microtubules during its migration. After fusion to produce the synkaryon, the conjugants separate.

Ultrastructure of Nuclear Division in Paramecium Aurelia III. Meiosis in the Micronucleus During Conjugation