SOME ULTRASTRUCTURAL FEATURES OF MICRONUCLEI DURING CONJUGATION AND AUTOGAMY IN PARAMECIUM-AURELIA

Micronuclei of P. aurelia at the prophase (crescent stage) of the 1st meiotic division during conjugation or autogamy were characterized by an elongated, somewhat curved shape. They contained small aggregates of a fibrillar material scattered uniformly throughout the nucleoplasm and a single layer of intranuclear microtubules located close under, and adhering to the inner nuclear membrane. These microtubules ran parallel to each other and parallel to the long axis of the micronucleus. At one end of the crescent stage micronuclei, there were 2 electron-dense bodies, which may function as organizing centers for microtubular assembly. During subsequent meiotic divisions the single layer of intranuclear microtubules was absent but a microtubular meiotic spindle was present. Both gamete pronuclei in each conjugant contained microtubules distributed evenly throughout the nucleoplasm and aligned in 1 direction. The migratory (male) pronucleus formed pseudopodia during its passage through the cytoplasmic bridge and was surrounded by extranuclear bundles of microtubules located in the cytoplasm close to the nuclear envelope. Stationary (female) pronuclei had no extranuclear microtubules. During all stages of conjugation or autogamy the nuclear envelope remained intact.