SIMON FRASER / SCIENCE PHOTO LIBRARY SIMON FRASER / SCIENCE PHOTO LIBRARY
Twin foetuses. Coloured magnetic resonance imaging (MRI) scan of a sagittal section through twin foetuses in the womb. The upper twin seems to have an abnormality (dark purple) in its brain. This could either be a cyst (a fluid filled lump), or an enlarged lateral ventricle (the fluid-filled spaces within the brain). Non-identical (dizygotic) twins develop when two separate eggs are released from the ovaries and are fertilised simultaneously. Identical (monozygotic) twins occur when the two cells formed by the first division of a fertilised egg develop independently.
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