A visible “blob” (or vacuole) - there are usually two of them - inside an egg that has undergone fertilization or activation by other means, and enclosing the chromosomes from the oocyte (the female pronucleus) and from the sperm (the male pronucleus). Plural: pronuclei. If three pronuclei are observed, the chances are that two sperm have entered the egg (polyspermy)
