The Origin and Development of the Lymphatic System. 21 



C. DIRECT BUDDING OF THE LYMPHATICS FROM THE VEINS. 



As has been stated, it was the study of the decreasing zones of lym- 

 phatic capillaries which led to the discovery that the first lymphatics 

 are sacs close to the veins. In mammalian forms the first lymphatics 

 are the anterior lymph sacs which bud off from the anterior cardinal 

 veins. These jugular lymph sacs were noted by Saxer (138) in 1896 

 in his extensive study on lymph glands, but he did not recognize 

 their significance. They can readily be made out in sections of pig 

 embryos 14-20 mm. long, or in sections of any mammalian embryo 

 of the corresponding stages. The fact that they are the first lymphatics 

 in mammals and that they lie close to the vein, separated only by the 

 double endothelial wall, led me to the conclusion that the lymphatics 

 budded directly off from the veins and it is gratifying to find that 

 this reasoning has been justified by further investigation (Sabin 129, 

 E. E. & E. L. Clark 29 and Hoyer 49a) . 



To follow the development of our knowledge in historical sequence 

 the next step was taken by F. T. Lewis (76) in a series of excellent 

 reconstructions of the anterior lymph sacs in rabbit embryos, the best 

 figures we have had up to the present time of the form of the early 

 sac. Lewis carried the study still further back and showed that the 

 definite sac was preceded by a blood-filled capillary plexus. This blood- 

 filled plexus has proved to be a very important observation. From the 

 presence of blood Lewis made the logical inference, with the evidence 

 at. his command, that these vessels were blood capillaries and that 

 hence lymphatics were transformed blood capillaries. The observation 

 of . the blood-filled vessels has been confirmed by Huntington and 

 McClure (54) on the cat and by myself in human and pig embryos 

 (132-133) and we have all accepted the theory of transformed capil- 

 laries until new evidence has now shown us the correct explanation 

 of the presence of the blood. 



This new evidence has been furnished by the work of E. K. and 

 E. L. Clark (27-29). E. L. Clark began with the study of the allan- 

 toic lymphatic vessels in chick embryos. From the allantoic vessels 

 she was led back to the posterior lymph hearts and the lymphatics of 

 the body wall. The injections of Mierzejewski (96) show that the 

 primary line of growth for the lymphatic vessels in the skin of the 

 chick is along the thoraco-epigastric vein (96, figs. 1 to 4). E. L. 

 Clark noted that in early stages the lymphatic vessels along this line 



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