The extraembryonic Mesoderm (EEM).
The EEM begins to develop by the end of Day 9 with the proliferation of an acellular epithelium between the exocoelomic membrane and the cytotrophoblast. The proliferated cell mass is called the extraembryonic mesoderm. This mesoderm surrounds the exocoelomic and amniotic cavities.
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Formation of the Yolk Sac
The exocoelomic cavity is now called the primitive yolk sac. (Note the prefix, extra, as in extraembryonic mesoderm, refers to structures that eventually develop into supporting tissues of the embryo and fetus i.e. amnion, chorion, and umbilical cord), while the prefix, intra, refers to the structures that eventually become part of the embryo and fetus. Thus, the extraembryonic mesoderm is eventually incorporated into the chorion.)
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Development of the Chorionic Cavity
The extraembryonic mesoderm increases in size and isolated spaces appear within it. These spaces will rapidly fuse to form a large cavity, the extraembryonic coelom or chorionic cavity. This fluid-filled cavity surrounds the conjoined amnion and primary yolk sac except just dorsal to the amnion where the extraembryonic mesoderm remains connected to both the amnion and the cytotrophoblast forming the connecting stalk which later becomes the major part of the umbilical cord.
The extraembryonic coelom splits the extraembryonic mesoderm into two parts, the extraembryonic somatic mesoderm, which lines the trophoblast and covers the amnion, and the extraembryonic splanchnic (or visceral) mesoderm, which covers the yolk sac. The extraembryonic somatic mesoderm and the two layers of trophoblast form the chorion, or fetal portion of the placenta. The chorion forms the wall of the chorionic sac within which the embryo and its amniotic and yolk sacs are suspended by the connecting stalk.
As the extraembryonic coelom forms, the primary yolk sac decreases in size and a smaller, secondary, yolk sac forms.
By the end of Day 13 and beginning of Day 14, the primitive yolk sac disintegrates into a collection of exocoelomic cysts. The much smaller secondary yolk sac is renamed the definitive yolk sac.
At Day 14, the embryo is still a bilaminar disc, but the hypoblastic cells are now columnar. In one area of the bilaminar disc, the hypoblastic cells form a thickened circular area called the prechordal plate. This plate indicates the future site of the mouth and is an important organizer of the head region.
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