The following lymph sacs are already present by the end of the embryonic period: the paired jugular (fused with the axillary), the paired lumbar and iliac lymph sacs, the unpaired retroperitoneal lymph sac at the root of the mesentery, the primordium of the cisterna chyli at the level of lumbar vertebrae 3 and 4, the thoracic ducts.
The thoracic ducts arise through fusion and enlargement of some isolated primordia. They can first be seen near the postcardinal veins in 16mm embryos, which corresponds to the end of week 6 or the beginning of week 7. In the 19mm embryo, connections with the nearby veins have been seen and so it is thought that the thoracic ducts develop from the postcardinal veins. They grow from the lower thoracic region in cranial and caudal directions. The right thoracic duct begins at the cisterna chyli, goes in a cranial direction, crosses the midline and finally connects into the left jugular sac. The left thoracic duct only grows up to the level at which the hemiazygos and the superior cava veins meet.
The right thoracic duct flows into the left venous angle (between the internal jugular vein and the subclavian vein). It develops valves during month 5. On the right side, the paratracheal and internal thoracic lymph networks give rise to the right bronchomediastinal duct. Together with lymphatic vessels around the jugular and subclavian veins, this forms the right lymphatic duct and ends in the right venous angle.