28o THE DEVELOPMENT OF THE VASCULAR SYSTEM 



posterior sacs; and to the mesentery from the retroperitoneal sac. According to 

 this view, then, endothelium can arise only from pre-existing endothelium. 



Other investigators (Huntington, Amer. Jour. Anat., vol. 16, 1914; Mem. 

 Wistar Inst., 1911; and McClure) hold that the lymph sacs are formed in situ by 

 the fusion of discrete mesenchymal spaces which become Hned with an endothe- 



FiG. 287. — Flat reconstruction of the primitive lymphatic system in a human embryo 30 

 mm. long (Sabin). X about 3.5. C.c, Cistema chyli; Lg., lymph gland; S.l.jug., jugular lymph sac; 

 SJ.mes., retroperitoneal lymph sac; S.l.p., posterior Ijonph sac; S.J.S., subclavian lymph sac; V.c, 

 cephalic vein; V.c.i., inferior vena cava; V.f., femoral vein; V.j.i., internal jugular vein; V.l.p; 

 deep lymphatics; V.I.S., superficial lymphatics; V.r., renal vein; V.s., sciatic vein; V.u.(J>.), primi- 

 tive ulnar vein. 



Hum of transformed, bordering mesenchymal cells. Venous connections are purely 

 secondary. The thoracic duct and the peripheral vessels develop similarly 

 by the progressive fusion of separate clefts; hence endothelium can differentiate 

 continually from young mesenchyma. The further growth of endothelium, already 

 formed, is not denied. 



