pronucleus polar body nucleus FBbt:00000051 Haploid nucleus formed by the two meiotic divisions following oocyte ovulation. There are 3 polar body nuclei, located in a single file roughly perpendicular to the dorsal surface and to the long axis of the embryo. The fourth, and most inner of the haploid nuclei, forms the female pronucleus. The polar body nuclei are surrounded by yolk-free cytoplasm. fly_anatomy.ontology