The lymphatic system begins to take form in the human embryo at approximately the sixth week of development. This primordial system appears as a collection of six lymph sacs: two jugular lymph sacs, two iliac lymph sacs, the cisterna chyli, and the retroperitoneal lymph sac. They all originate from the mesoderm and are extensions of veins, being similarly lined with endothelium. Eventually, the lymphatic sacs give rise to a network of lymph nodules interconnected via endothelium-lined channels, establishing the early lymphatic system in the human embryo.