The fusion of sexual nuclei.
D000818:Animals; D002467:Cell Nucleus / Q000502:physiology*; D003594:Cytoplasmic Granules / Q000502:physiology; D005306:Fertilization; D008561:Membrane Fusion; D008854:Microscopy, Electron; D009685:Nuclear Envelope / Q000502:physiology* / Q000648:ultrastructure