Human Embryogenesis, Fertilization And Fetus Development

Cytotrophoblast cells and cells of Heuser's membrane continue secreting extracellular matrix between them. This matrix is called the extraembryonic reticulum. Cells of the epiblast migrate along the outer edges of this reticulum and form the extraembryonic mesoderm, which makes it difficult to maintain the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the chorionic cavity or extraembryonic coelom.
Another layer of cells leaves the hypoblast and migrates along the inside of the primary yolk sac. The primary yolk sac is pushed to the opposite side of the embryo (the abembryonic pole), while a new cavity forms, the secondary or definitive yolk sac. The remnants of the primary yolk sac are called exocoelomic vesicles (exocoelomic cysts).