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By the 13th day of development, the trophoblast has completely invaded the endometrium. The embryoblast has divided into two layers and the extraembryonic mesoderm has split into a somatic layer and a splanchnic layer. Since these mesodermal layers line a cavity that is located outside of the embryo proper, the cavity is called the extraembryonic coelom. The extraembryonic coelom is also called the chorionic cavity. Soon, after the formation of mesoderm within the embryo a mesodermally lined cavity will develop within the body of the embryo, the intraembryonic coelom. Initially, the intraembryonic and extraembryonic coeloms are in communication