Eight-week-old foetus. This foetus is seen during the end stage of the first trimester. It is attached to the placenta and the mother's blood circulation by an umbilical cord (far left). The sac surrounding the foetus is known as an amniotic sac, which is filled with amniotic fluid. At upper right are the remnants of the yolk sac. The embryo's eye and limbs are visible, as is its 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. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR M.A. ANSARY)
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