The Origin and Development of the Lymphatic System. 77 



There are numerous points which rule out the theory of the origin 

 of the lymphatics by extra-intimal spaces. (1) These spaces can be 

 varied at will by changing the fixation. In my experience they are 

 much more common with lymphatics than with veins. We have sec- 

 tions of adult human . tonsils, in which all surrounding lymphatics 

 have extra-intimal spaces. In the human embryo, 460, the large 

 jugular segment of the thoracic duct has one area where the endo- 

 thelium has sagged from the surrounding tissue. There is, I think, 

 no question but that the extra-intimal spaces are artefacts. This is 

 made the more certain by the fact that they do not occur in the living 

 tadpole's tail, but are found along the veins after fixation. 



(2) In connection with the presentation of the extra-intimal 

 spaces, Huntington gives no proof whatever that the structures he 

 figures are degenerating veins. Most of the extra-intimal spaces he 

 shows are along the line of the pulmonary and cardiac lymphatics 

 arising from the jugular sacs. I think it probable from their size 

 and position that the structures he is dealing with are shrunken 

 lymphatics and not veins. The pulmonary and cardiac lymphatics 

 (fig. 13) will, when reconstructed, give the same appearance of iso- 

 lated lymphatic vessels as all other lymphatic trunks. 



(3) The third point against the theory that lymphatics grow by 

 extra-intimal spaces is this: the growing lymphatic tip always keeps 

 as far as possible from the blood capillaries, just as in the adult the 

 ultimate lymphatic capillaries are as far as possible from the blood 

 capillaries; as, for example, in the relation of the central lacteals 

 and peripheral blood capillaries in the villus. The point of the avoid- 

 ance of the blood capillaries and of the mesenchyme cells as well by 

 the growing lymphatic tip has been well described by Clark (26). 

 Moreover, in certain tadpoles the lymphatics grow out ahead of the 

 blood capillaries where no blood capillaries have ever been. Therefore, 

 lymphatics do not grow by extra-intimal spaces, for extra-intimal 

 spaces are artefacts; there is no evidence that lymphatics follow 

 degenerating veins and on the other hand the growing lymphatic 

 tip, far from following the blood capillaries, avoids them as much 

 as possible. 



3. Growth of Lymphatics by the Addition of Tissue Spaces. 



With these various structures as anlagen of lymphatic capillaries, 



Huntington and McClure have included certain of the tissue spaces 



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