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Embryonic Period
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Description
About the 12th day after fertilization, the extraembryonic mesoderm develops.
The cells form a connective tissue layer around the amnion and yolk sac.
Large cavities develop that fuse and form the extraembryonic coelom.


The extraembryonic mesoderm together with the trophoblast forms the chorion which surrounds the embryo and, later, the fetus. The chorion
Blocks antibody production by the mother
Promotes production of T lymphocytes to suppress the immune response in the uterus
Produces human chorionic gonadotropin (hCG)


The first major even of the 3rd week of development is gastrulation.
The two-layered embryonic disc transforms into a trilaminar (three-layered) embryonic disc (ectoderm, mesoderm, endoderm)
Gastrulation is associated with the rearrangement and migration of cells from the epiblast.
The first step in gastrulation is formation of the primitive streak.

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