CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY
Human embryo. Illustration of a human embryo aged 3 weeks (21-22 days) seen in the uterus. Measuring about 3 millimetres in length, the embryo is the dumb-bell shaped structure at centre right. Its head region (upper centre) is divided into two neural bulges. In the middle of the embryo are six pairs of segmented somites which will form voluntary muscle, bones and connective tissue. At this early stage the embryonic nervous system is forming. The embryo floats within its amniotic sac (brown), attached to the placenta (at lower left) by an umbilical cord. A yolk sac (yellow, centre left) supplies nutrients to the embryo.
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