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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.