The Lymphatic System in Human Embryos. 57 



seen in sections. But in these specimens of the entire living tail, 

 endothelium can be distinguished from mesenchyme, and the lym- 

 phatics grow out from their own endothelium and do not add any 

 peripheral anlagen. 



A second point which Dr. Clark observed, but did not publish, is 

 the sudden collapsing of a part of a lymphatic vessel. Once 

 or twice while a red blood cell was pushing its way into the 

 vessel, its central end collapsed suddenly to an endothelial thread, 

 while the peripheral end remained dilated. These collapsed lym- 

 phatics right in the middle of a vessel were noted and figured by 

 Langer; they have been noted many times in blood capillaries, for 

 example, see Fig. 49 of Strieker's Handbuch der Histologic, but 

 they have been interpreted as evidences of growth simply, while it 

 may be that these collapsed vessels are a part of the functional 

 activity of the lymphatic capillaries. The reverse of this process 

 of collapsing of the vessels, namely the sudden opening up of tiny 

 vessels during an injection, I have often observed and used as an 

 argument in favor of continuous lymphatics rather than isolated 

 anlagen. ( See Symposium on the Lymphatic System. Anat. Record, 

 Vol. II, 1908.) It can be readily seen that in cross section these en- 

 tirely collapsed vessels might be wholly lost and only the dilated por- 

 tions shown, and thus the suggestion is that Lewis' anlagen represent a 

 transitory phase of the functional activity of the lymphatic capil- 

 laries. 



The question of the multiple anlagen has resolved itself wholly 

 into a question of method, with the study of the living lymphatics 

 and injected lymphatics on the one hand and the method of serial 

 sections on the other. Ludwig's famous phrase, "die Methode ist 

 alles," was never more apt than in this connection, for it sums up the 

 whole situation. Having long worked with injections we are con- 

 vinced that uninjected serial sections are wholly inadequate to show 

 all the blood capillaries or lymph capillaries, moreover, we are con- 

 vinced that Dr. Lewis has carried the observations as far as they 

 can be carried with sections and that sections will always show these 

 apparently isolated anlagen. To put the contention that serial 

 sections are inadequate to the test. Dr. Clark has made the following 



