In spite of the vast amount of work which has been done during the past thirty years 011 the development of the lymphatic system, there are still many gaps in our knowledge of it. Previous to 1900, no conclusive studies had been made regarding the question, although several hypotheses had been proposed, which may be classified in two general groups. According to one view, lymphatic endothelium was thought to be a derivative of blood-vascular endothelium, while according to the second and more widely held view lymphatic endothelium differentiated from mesenchyme cells and acquired connections with the vascular system secondarily. The growth of the differentiated system was generally considered to take place by the coiitinued addition of mesenchyme cells, although Ranvier ('97) pictured lymphatic endothelium as invading the body by a process of out sprouting, as the roots of a tree extend into the ground, while S. Mayer ('85) believed he had evidence for the continued transformation into lymphatic vessels of blood vessels cut off from the remainder of the blood-vascular system. A new era in our knowledge of the lymphatic system was inaugurated in 1901 by FR Sabin's epoch-making studies