ORIGIN OF LYMPHATICS IN BUFO 179 



modified. If we are to have agreement in our accepted belief 

 of the mode of lymphatic genesis, the two chief antagonistic 

 schools of investigators are compelled to re-examine their material 

 with a new unbiased attitude, both those who maintain that 

 lymph ducts are formed in situ from mesenchyme and discontinu- 

 ously, and those who contend for the view of the venous origin 

 of such vessels, of their continuity in development and of their 

 centrifugal growth and spreading from a few definite foci in the 

 body. The idea that strikes the writer primarily is that most 

 workers have been deceived as to the time when lymphatic anlagen 

 first make their appearance; it is conceivable how structures 

 which have been described as the incipient anlagen of a lymph 

 duct may already represent a much later phase. The char- 

 acteristics which the lymphatic rudiments, discussed in this 

 paper, manifest, such as solidity or imperfect vacuolation and 

 discontinuity, though venous in origin, irrefutably show that the 

 injection method would have been utterly incapable of revealing 

 them at this time. Might not the same contention apply to 

 the study of lymphatics in other vertebrate embryos, and might 

 not anlagen exist long before injections could attest their existence. 

 Is most of the other work on lymphatic development exempt 

 from similar criticism? In their suppositions investigators have 

 been more or less led astray by appearances, and in no other 

 field of inquiry perhaps do appearances intimate so little of the 

 truth. In a previous paper on the development of the thoracic 

 duct in the pig embryo, the writer has described and figured ver}^ 

 definite spaces which lie in the vicinity of the cardinal veins or 

 their tributaries and in the path of the future definitive duct, 

 and which he believed to be lined by cells mesenchymal in de- 

 rivation. This conclusion was reached because the intimal 

 cells of the thoracic duct anlagen at their inception visibly re- 

 sembled the embryonic connective tissue elements very closely 

 in certain seemingly important qualities. In finding that the 

 character which marks incipient endothelium from mesenchyme 

 particularly in the head region of j^oung toad larvae is its abund- 

 ance of yolk while other cell attributes appear identical, the 

 present inquiry has convinced him of other potent possibilities. 



