The Lymphatic System in Human Embryos. 47 



The thoracic duct, which he found extended only from the beginning 

 of the coeliac artery to the outlet of the superior vena cava, began 

 on the eighth day in the following manner. First, a series of mesen- 

 chyme spaces around which occur clumps of mesenchyme cells which 

 develop into a solid cord. These solid cords become excavated and 

 form the thoracic duct. There is nothing to correspond with this 

 in connection with the lymphatic system in mammals. 



To trace the development of the idea that the lymphatic system 

 is derived from the venous system it is necessary to begin with the 

 work of Langer,^ published in 1868. 



In this important paper, Langer makes clear a number of funda- 

 mental points. He distinguished the lymphatics in the tadpole's tail 

 from the arteries and the veins by injecting them. He found the 

 two longitudinal lymphatic vessels of the tail, and the branches 

 forming a plexus leading from them. He distinguished the lym- 

 phatic vessels clearly from the surrounding connective tissue, and 

 determined that the lymphatics were closed tubes. He was studying 

 a border zone of developing lymphatics and saw that the lymphatics 

 here were really terminal blind ends. He noted the endothelial 

 sprouts from the sides and ends of the vessels and interpreted 

 these sprouts to mean that the lymphatic vessels grow by the same 

 method as do blood capillaries. 



Thus he says: "Ich zweifle nicht, dass Lymph und Blutcapillaren 

 nach dem einen und demselben Bildungsmodus sich vermehren, die 

 Elemente sind dieselben." This in reality is his great contribution 

 and upon this idea as a foundation rests the new conception of the 

 lymphatic system as derived from the veins. Another of his obser- 

 vations must not be omitted, namely that in the course of a lymphatic 

 capillary, a portion of the vessel may be greatly narrowed, that is to 

 say, even completely collapsed. "Ich traf aber auch Rohrchen, 

 welche sich ziemlich rash verengten und in der Mitte ihres Verlaufes 

 einen diinnen, anscheinend ganz soliden Faden darstellten." The 

 meaning of this phenomenon and its relation to the general theory 

 will be made clear later. 



°Langer. Ueber das Lymphgefasssystem des Frosches. Sitzb. d. k. Akad. 

 d. Wissensch., LVIII Bd.. I Abth., 1868. 



