Light micrograph cross section through a pair of eggs of a Giant roundworm (Ascaris lumbricoides) showing interior cells undergoing mitosis. Mitosis is part of the cell cycle when replicated chromosomes are separated into new nuclei. When an embryonated egg is ingested, the larva hatches then penetrates the wall of the gastrointestinal tract and enters the blood stream. This very large worm is a parasite of both animals and humans.