Once the embryonic body has acquired its third dimension following cranial and caudal flexions, it is covered with surface ectoderm up to the umbilical ring. In addition, the transition from surface ectoderm to amnion is shifted in a ventral direction towards the umbilical ring. The amnion then covers the umbilical cord (tubular part of the amnion), and from that point on encloses the entire embryo. The umbilical vesicle remains in the chorionic cavity. The embryo becomes surrounded by three concentric cavities: first by the amnion, then the chorionic cavity, and finally by the uterine cavity.