HENNING DALHOFF / SCIENCE PHOTO LIBRARY HENNING DALHOFF / SCIENCE PHOTO LIBRARY
Gastrulation during embryonic development. Illustration showing part of the process of gastrulation, the stage of embryonic development that follows implantation of the blastocyst and comes before the development of organs (organogenesis). At the gastrulation stage (around 2-3 weeks after fertilisation), the cells of the newly forming embryo (gastrula) are differentiating into the three germ layers: the ectoderm, endoderm and mesoderm. The ectoderm forms epidermis and nervous tissue, the endoderm forms the digestive tract, glands and lungs and the mesoderm forms the connective tissue, including blood vessels and cartilage. Here, the gastrula (orange) is within the chorionic cavity (red) that is implanted in the uterus wall (left). The embryonic disc (vertical, centre) lies between the amniotic cavity (left) and the secondary yolk sac (right).
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