August 4, 1916] 



SCIENCE 



151 



the neck, and these primary lymph glands 

 bear a definite relation to the secondary 

 glands which form along the ducts growing 

 out from the sac. 



At a slightly later stage — in embryos of 

 the seventh week, approximately 20 mm. in 

 length — a series of lymphatic buds develop 

 from some of the abdominal veins. These 

 early buds have proved more difficult to 

 study than the jugular buds — first because 

 the veins from which they arise are more 

 complex and were less well known, and sec- 

 ondly because their deep position has made 

 direct observation in the living embryo and 

 direct, precise injections practically impos- 

 sible. Therefore our knowledge of the ex- 

 tent and origin of the abdominal lymphatics 

 from different veins is still far from com- 

 plete. Certain very interesting observations 

 by Silvester 11 on monkeys and by Job 12 on 

 rats show that in these forms certain lym- 

 phatic ducts drain permanently into the 

 inferior vena cava, the iliac, the renal or 

 the portal veins, suggesting a multiple 

 origin of lymphatics from the abdominal 

 veins. The main abdominal lymphatics 

 begin as a retroperitoneal sac which devel- 

 ops from a vein connecting the two Wolffian 

 bodies. This vein ultimately forms a part 

 of the inferior vena cava. This large retro- 

 peritoneal sac furnishes the key for the 

 study of the abdominal lymphatics. The 

 lymphatics of the skin of abdomen and for 

 the legs grow from paired iliac sacs. The 

 retroperitoneal sac and the paired iliac sacs 

 become connected with the left jugular sac 

 by means of the thoracic duct, which grows 

 from the left jugular sac and from the ab- 

 dominal lymphatics, and is complete in 

 embryos about 25 mm. long. There is thus 

 formed a primary lymphatic system of sacs 

 connected by the thoracic duet; this sys- 



11 Silvester, C. F., Amer. Jour, of Anat., Vol. 12, 

 1911-12. 



is Job, T. T., Anat. Beeord, Vol. 9, 1915. 



tern in mos: mammals drains into the 

 internal jugular veins on either side. From 

 the primary sacs, a plexus of capillaries in- 

 vades the body. In a general way, the 

 vessels from the jugular sacs grow to the 

 head, thorax and thoracic viscera; those 

 from the retroperitoneal sac to the abdom- 

 inal viscera, and in part to the thoracic 

 viscera; and those from the iliac sacs to 

 the abdominal walls and legs. 



The injection of these invading plexuses 

 of lymphatics from the sacs outward is 

 possible in the embryo, though it is im- 

 possible in the adult, owing to the fact that 

 the early vessels are without valves. In a 

 general way it may be stated that by the 

 time a fetus has reached the length of 5 cm. 

 almost the entire skin has been invaded by 

 a single plexus of lymphatic capillaries and 

 the organs have received their primary lym- 

 phatic vessels. At this stage of embryonic 

 development injections of any part of the 

 lymphatic plexus spread out in all direc- 

 tions, so that theoretically the injection of 

 any capillary might fill the entire system. 

 I have injected the thoracic duct, for ex- 

 ample, from the skin of the thorax, the in- 

 jection mass passing around through the 

 iliac lymphatics; or again I have injected 

 the lymphatics of the skin by puncturing 

 the thoracic duct. This complete anastomo- 

 sis of the primary lymphatic capillary 

 plexus of both the superficial and the deep 

 systems in the embryo seems to me to be of 

 considerable importance. 



To illustrate the development of the lym- 

 phatic system to an organ and without an 

 organ, I shall describe Cunningham's 13 

 work on the lymphatics of the lung. He has 

 found that lymphatics approach the lung 

 from three sources — from the two jugular 

 sacs there are right and left lymphatic 

 trunks and from the retroperitoneal sac 



13 Cunningham, E. S., ' ' Proc. Amer. Asso. of 

 Anat., ' ' Anat. Record, Vol. 9, 1915. 



