Blood islands are structures around the developing embryo which lead to many different parts of the circulatory system.
Blood islands arise external to the developing embryo on the umbilical vesicle, allantois, connecting stalk and chorion.
Mesenchymal cells exterior to this form the muscular and connective tissue components of blood vessels.
They develop from undifferentiated hemangioblasts in blood vessels in the walls of the umbilical vesicle, allantois and chorion.
Over time, the network will continue to grow, and will eventually penetrate the embryo proper and fuse with the concurrently developing embryonic circulation.