DR G. MOSCOSO / SCIENCE PHOTO LIBRARY DR G. MOSCOSO / SCIENCE PHOTO LIBRARY
Human embryo. View of a human embryo at 7-8 weeks old, known as a first trimester embryo under the age of 12 weeks. The embryo is attached to the placenta and the mother's blood circulation by an umbilical cord (upper left), and is seen floating in an amniotic sac filled with amniotic fluid. At upper right is the remnant of the yolk sac. The embryo's eye and limbs are visible, as is its male sex. During the first trimester the embryo develops 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.
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