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Eight-week-old foetuses. Four coloured images of a human foetus. They are attached to the placenta and the mother's blood circulation by an umbilical cord (left of each foetus). The sac surrounding the foetuses is known as an amniotic sac. The embryos' eyes and limbs are visible, as are their male sex. During the first trimester, embryos develop a distinct human appearance. In this early period, organ differentiation overshadows growth and all the major organs have been formed. At this age the embryo is about 4 centimetres in length and less than 10 grams in weight. The images are coloured in the style (pop art) of Andy Warhol. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR M.A. ANSARY)