Pronuclear membrane alterations during approximation of pronuclei and initiation of cleavage in the rabbit.

As the male and female pronuclei of flushed rabbit tubal ova approach each other, numerous projections appear on the proximal surfaces, resulting in a severely irregular appearance. Each pronuclear envelope consists of 2 parallel membranes separated by a distance of 40-60 nm; annular ' pores' are conspicuous in regions of pronuclear approximation. Stacks of smoothsurfaced membranes form parallel annulate lamellae in the cytoplasm adjacent to the pronuclei. When the 2 pronuclei come to lie in direct apposition, the adjacent envelopes are flattened against one another, crowding out intervening cytoplasm and producing an arrangement of 4 equidistant membranes. Interspersed between segments of apposing membranes are small 'cytoplasmic islets'. At several points along the approximating surfaces internuclear channels form between adjacent projections. This results in partial pronuclear union, but not complete fusion. After disruption of the pronuclei and the appearance of chromosomal aggregates in the centre of the egg, segments of pronuclear membranes are still visible in the cytoplasm. The membrane fragments are associated with collections of smooth vesicles, the latter increasing greatly in number during cleavage.