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Identifying and sequencing stages of human embryonic development. An embryo is a multicellular eukaryotic organism in its early stages of development. In humans, an embryo is carried in the uterus. In humans, the embryo is a developing organism until the first eight weeks, after which it is called a fetus until birth. A fertilized an egg cell is called the zygote, which contains DNA of the two parents. The zygote will begin to divide by mitosis. The result is an embryo. Three stages of embryonic development 1. Blastula 2. Gastrula 3. Organogenesis The blastula stage features a fluid-filled cavity surrounded by a sphere or sheet of cells. During the gastrula stage, cells of the blastula form three layers the ectoderm, the mesoderm and the endoderm. The ectoderm will become the cells of the skin and other surface cells. The mesoderm will become the cells of connective tissue and the endoderm will become the cells of the inner organs. During organogenesis, cells differentiate into specific cell types. The ectoderm cells will further separate into skin cells, and cuticle cells and hair cells. The mesoderm will further divide into tendons, and muscles and bone. And the endoderm will divide into kidneys and heart and spleen. Timeline • Week 1-4 5-7 days after fertilization, the blastula attaches to the wall of the uterus. • The embryo's growth centers around an axis, which will become the spine and spinal cord. • Week 5-6 The heart will begin to beat. • Limb buds appear where the arms and legs will grow later. • The head represents more than half of the embryo's mass. • Week 7-8 The embryo's blood type becomes apparent. • Embryo is capable of motion, and the eyes begin to form. At the end of the 8th week, the embryonic stage is over, and the fetal stage begins.