A sperm factor as the counterpart to the sperm-binding factor of the homologous eggs.
D000818:Animals; D005260:Female; D005306:Fertilization; D008297:Male; D010053:Ovary / Q000502:physiology*; D012617:Sea Urchins / Q000502:physiology; D013084:Sperm-Ovum Interactions; D013094:Spermatozoa / Q000502:physiology*