Beginning in the third week and continuing into the fourth week, the embryo starts to change form. It transforms from a disc shape to a cylindrical shape largely through folding in the sagittal plane and the transverse plane. Due to an increase in the length of the embryo, the cranial and caudal regions of the embryo move inferiorly. At the same time, the rapid growth of the neural tube and somites causes the lateral aspects of the embryo to fold inward. The net effect is a closing off of the body cavity ventrally around a point centered on the umbilical cord.
In the animation observe what appears to be a blue bubble sitting on top of a yellow bubble. The blue bubble is the surface ectoderm covered by the amnion. The yellow bubble is the endodermally lined yolk sac. Note the presence of the connecting (body) stalk which consists of extraembryonic mesodern that attaches the embryo to the chorionic sac. At the end of the animation, the folding of the embryo has squeezed the yolk sac against the body stalk these structures will come to be located within the umbilical cord
In this animation note how the lateral plate mesoderm splits to create the intraembryonic coelom which is ultimately incorporated into the embryo. Note that the gut tube is suspended from the posterior body wall by a sheet of mesoderm which will become the dorsal mesentery. Observe that there is a ventral mesentery initially, but it disappears from all but the foregut region of the gut tube.