SOVEREIGN, ISM / SCIENCE PHOTO LIBRARY SOVEREIGN, ISM / SCIENCE PHOTO LIBRARY
Twin foetuses. Coloured X-ray of full-term twin foetuses in the womb. The foetus' bones are grey and the mother's bones are orange and green. One foetus' skull is at bottom centre with its spine curving upwards and to the right. The other foetus' head is at left centre with the spine curving upwards and to the right. The mother's spine runs down centre, with her pelvis at bottom. Non-identical (dizygotic) twins develop when two separate eggs are released from the ovaries and are fertilised simultaneously. Identical (monozygotic) twins occur when two cells formed by the first division of a fertilised egg develop independently.
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