Figure 1.

The oocyte meiotic divisions and early embryogenesis. Oocyte meiotic maturation involves nuclear envelope breakdown (meiotic M-phase entry) and cortical rearrangement. The oocyte is fertilized as it enters the spermatheca. The condensed sperm chromatin is shown in the posterior (at right) of the embryo. The MI and MII divisions are completed in the uterus, forming two polar bodies at the anterior. Pronuclei form (bottom left), meet in the posterior, and fuse as they enter mitotic M phase (the small unfilled circles are centrosomes). The time from nuclear envelope breakdown to pronuclear meeting is ∼45 min.