In human embryonic development, the amnion and yolk sac develop early on, with the extraembryonic coelom and sinusoids developing later through the proliferation and differentiation of cells. The extraembryonic coelom is also referred to as the chorionic cavity and is depicted in a drawing of the blastocyst embedded in this text. This development occurs during the third week of development and is an important process in the formation of the embryo. The key points to take away are the early presence of the amnion and yolk sac, the later development of the extraembryonic coelom, and the role of cell proliferation and differentiation in these processes.