Distinct collections of cells called blood islands develop in the outer mesoblastic layer of the yolk sac, especially on its ventral surface. The blood islands produce the primitive blood cells. Many endothelial-lined spaces filled with primitive blood cells form in the mesoblastic layer. These spaces will soon become continuous with those forming in the connecting stalk, chorion and embryo proper. They represent the first appearance of the Cardiovascular System.
AMNION
The two-layered amnion arches over the dorsal and lateral aspect of the embryonic disc as the lateral body folds develop.
CONNECTING STALK
The connecting stalk bends at nearly right angles to the embryonic disc. It is wedge shaped, being narrow near the embryo and broad at its attachment to the chorion. The most advanced blood vessels are in the stalk near the embryo. These vessels are precursors of the umbilical vessels.
ALLANTOIS
The allantois becomes a distinct, endodermally lined duct caudal to the cloacal membrane.
Source: Atlas of Human Embryos.