Work on the lymphatic system began in the 17th century, and by the beginning of the 19th century the anatomy of most of the lymphatic system had been described. One of the most important questions in this field has been the determination of the embryological origin of the lymphatic endothelium. Two theories were proposed. The first suggested that lymphatic endothelium derived by sprouting from venous endothelium, the so-called centrifugal theory. The second, the so-called centripetal theory, suggested that lymphatic endothelium differentiates in situ from primitive mesenchyme, and secondarily acquires connection with the vascular system. More recent evidence has provided support for both hypotheses.