Ultrastructure of meiosis and polar body formation in the egg of the mud snail, Ilyanassa obsoleta.

Ilyanassa eggs pinch off two polar bodies at the animal pole prior to the first cell division. During prophase and early metaphase the meiotic apparatus is separated from the cortex by a layer of lipid droplets and can be found in any orientation relative to the cell surface. As the astral microtubules grow, the meiotic apparatus appears to be pushed toward the animal cell surface by the expanding internal aster. By late metaphase and early anaphase the peripheral aster first flattens and then appears to be folded down around the spindle, as the peripheral half-spindle is extruded out of the egg. The internal aster and the spindle remain intact at telophase, while the peripheral aster disassembles. A cortical fine filamentous layer is present after early metaphase into which peripheral aster microtubules enmesh. Such an interaction may provide the means of attachment of the meiotic apparatus to the cell cortex. A band of microfilaments is organized around the neck of the forming polar body, and it is thought to act as a contractile ring. It is unlikely that the stimulus for meiotic cytokinesis is like that established for mitotic cytokinesis in eggs; i.e., the region of astral overlap during metaphase determines the site of the furrow, since during meiotic metaphase the region of astral overlap is in the cytoplasm well below the surface. The cleavage furrow does bisect the spindle.