THE DEVELOPMENT OF LYMPH VESSELS 231 



1912, and others) interpret these sacs as the products of fusions of still 

 more primitive discrete lymphatic anlages which arose as mesenchymal 

 spaces; and their connection with the subclavian, sciatic and renal veins 

 as secondary unions. Sabin and others regard the lymphatic endothelium 

 once having sprouted from the venous endothelium as strictly specific, and 

 the entire lymphatic system as a derivative by sprouting and fusion of 

 these three sets of anlages. 



Huntington and McClure derive the definitive lymphatic system by 

 a progressive fusion of isolated mesenchymal spaces (mainly in the extra- 

 intimal portion of disappearing veins) and cells in the paths of the 

 future lymphatic trunks. According to the one school lymphatic endo- 

 thelium can arise only by proliferation of preexisting endothelium; accord- 

 ing to the other, endothelium can continually differentiate from young 

 mesenchyma. 



The recent observations of Clark (Anat. Rec., 3, 4, 1909) on the 

 growing lymphatics in the tail of living frog tadpoles where the process 

 of sprouting could be clearly followed, leaves no room for doubt that 

 lymphatics spread through sprouting, but the material and data give no 

 information as to the manner of origin of the initial anlages, which is 

 the real question at issue. It is perhaps as yet too early to decide the mat- 

 ter on the basis of available evidence, but the extensive histologic data of 

 Huntington and McCluro strongly support their claim of primary lym- 

 phatic origin by confluence of isolated mesenchymal spaces. 



The beautiful injections of Miss Sabin which show a progressively 

 enlarging continuous system in pig embryos apparently flatly contradict 

 this hypothesis; but the objection cannot be fairly ignored that the advo- 

 cates of lymphatic origin through fusion of isolated spaces base their 

 claims on appearances before the establishment of a continuous system 

 or even the several sets of lymphatic sacs, and the further fact that the 

 injection method is unsuitable for revealing lymphatic anlages existing 

 as isolated spaces. As concerns endothelium in general, Huntington 

 (Amer. Jour. Anat., 16, 3, 1914) regards an endothelial cell as simply 

 an adaptive form of a mesenchymal cell, 'modified in accordance with 

 definite hydrostatic and other purely mechanical factors,' resulting from 

 the presence of blood or lymph. On drainage of the fluid and consequent 

 release of pressure, the endothelial cell is believed to be capable of again 

 reverting to 'the type of the indifferent mesenchymal cell.' Kampmeier 

 (Amer. Jour. Anat, 17, 2, 1915) presents evidence from a study of sec- 

 tions of the young toad embryo apparently demonstrating the primary 

 origin of lymphatic endothelium only from venous endothelium; but the 

 primary sacs and ducts arise by a confluence of these earlier discrete 

 venous buds. A concise discussion and summary of this subject is given by 

 McClure (Anat. Rec., 9, 7, 1915.) 



