Early embryonic development. Illustration showing an early stage of the development of a human embryo, just after gastrulation, with the newly forming embryo at centre (pink elongated structure). The embryo and its associated amniotic sac and yolk sac are within the chorionic cavity (red) that is implanted in the uterus wall (left). The embryonic spinal cord is forming, with the yolk sac (pink, upper centre) still attached to the embryo. This yolk sac will later be absorbed into the developing digestive system. The embryo is in the amniotic sac (grey, lower centre). This is the start of the organogenesis stage, around 3 weeks after fertilisation. The cells of the newly forming embryo have differentiated into the three germ layers, and are now beginning to organise to form the different organs and body systems.
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