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The extraembryonic mesoderm in human embryos is believed to form from the hypoblast (although trophoblast contribution is also plausible), while in mouse, it arises from the caudal end of the primitive streak.
The extraembryonic mesoderm fills the space between the trophoblast and the amnion and the chorion.
After the primary yolk sac is displaced by the definitive yolk sac (12 dpc), large cavities form within the extraembryonic mesoderm, which becomes confluent and forms the extraembryonic coelom cavity, thereby effectively splitting the mesoderm into two layers: the extraembryonic mesoderm, lining the chorion and amnion, and the extraembryonic mesoderm, covering the yolk sac.
By day 13, the embryonic disc remains connected to the chorion only via the connecting stalk of extraembryonic mesodermal origin (which develops into the umbilical cord). The extraembryonic mesoderm also contributes to the formation of lymph, endothelium and blood.