A study of microscopical serial sections of 40 well-preserved human embryos, ranging in age from about 40 to 65 days (8-33 mm C.-R. length), revealed that the prevertebral main lymphatics develop from a number of separate primordia, all derived from venous walls. These primordia are: paired jugular and axillary lymph sacs and paratracheal, internal throacic, lumbar and iliac lymph plexuses, and single subtracheal and mesenteric lymph plexuses. Probably, the thoracic ducts develop from a series of small additional primordia. All these primordia, except the jugulo-axillary lymph sacs, lose their connections with the veins. They rapidly enlarge, fuse with one another and send out sprouts into peripheral regions, so that soon the definitive pattern of the major lymphatics can be recognized. During development some variations may occur, in particular in the area where the lower extensions of the two throacic ducts meet the lumbar lymph plexuses. Usually, the right thoracic duct primarily comes into contact with the jugulo-axillary lymph sac. The conclusion is drawn that on an embryological basis in the prevertebral region additional lymphatico-venous communications might be expected, but that possible lymphatico-venous communications elsewhere in the body always point to developmental aberrations or to pathology.