Twin foetuses. Coloured magnetic resonance imaging (MRI) scan of 32-week-old twin male foetuses. The placenta (grey) is seen at lower left. This stage of development is known as the third trimester and it is when the foetus develops its own immune system. It also begins to open its eyes during alert times and move them during periods of dream sleep. Non-identical (dizygotic) twins develop when two separate eggs are released from the ovaries and are both fertilised. Identical (monozygotic) twins occur when the two cells formed by the first division of a single fertilised egg develop independently.