76 Florence R. Sabin. 



by which all other vessels are formed, and the term fenestration, 

 giving the vague suggestion of the splitting of the wall of a vein 

 once formed, does not describe the actual process and is misleading. 

 We now know that the jugular sacs bud directly from the anterior 

 cardinal vein as lymphatics and grow by the sprouting of their endo- 

 thelium. 



As has been said, Huntington and McClure believe that the jugular 

 lymphatics come from the veins, but that they remain only as a means 

 of communication between the veins and the ' lymphatic ducts-. The 

 lymphatic ducts they think develop in a variety of ways. 



The entire question of the method of growth of the lymphatic 

 system is now concentrated on a study of the methods of growth of 

 the thoracic duct. It is probable that the thoracic duct arises in two 

 places; that it is formed in mammals by an asymmetrical down- 

 growth from the left jugular sac and by a plexus which arises sym- 

 metrically from the renal veins and grows along the course of the aorta. 

 This second portion arises later than the first and is in the blood-filled 

 stage (pig 23 mm.) when the jugular stalk is empty. These two anlagen 

 grow as do all other lymphatics and join by the same process by which 

 any lymphatic plexus is formed. 



The opposing views are brought out in two papers, one by Hunting- 

 ton (58) and one by Kampmeier (66). Huntington describes a compli- 

 cated method of origin of the thoracic duct, including (1) a jugular 

 stalk; (2) extra-intimal spaces, and (3) general tissue spaces. Kamp- 

 meier, on the other hand, gives a clear and excellent presentation of 

 the theory of the origin of lymphatics by the addition of tissue spaces. 



I will begin with the work of Huntington (58). His memoir on 

 the peripheral lymphatics has excellent photographs of sections, so that 

 it is easy to see what he is considering as lymphatics. For example, 

 figs. 1 to 9 are tissue spaces, figs. 10 to 12 are extra-intimal spaces, 

 and figs. 13 to 19 are probably lymphatics. Figure 25 shows the iliac 

 sac labeled 76. Numerous examples of extra-intimal spaces are shown 

 in figs. 105 to 147. Perhaps the best figure of an extra-intimal space 

 is Xo. 111A. Figures 148 to 158 are lymphatics. 



The theory of the origin of lymphatics from extra-intimal spaces 

 has been especially described in an article by Huntington in 1910 

 (55). In a diagram on page 409 he gives his idea that -a space which 

 forms around a degenerating vein eventually includes the entire vein, 

 which then disappears so that the lymphatic is left with a wall of 

 mesenchyme. 



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