GENERAL CONSIDERATIONS 67 



spaces, we shall again be left without a clue as to the origin of the 

 lymphatic system. 



It is significant to note that, although Sabin considers these 

 isolated endothelial-lined anlagen of Lewis as having been 

 detached from the veins, she nevertheless now recognizes their 

 existence in the pig embryo, and regards them as entering into 

 the formation of the thoracic duct ('11, p. 424). 



The point I wish to emphasize in this connection is that Sabin 

 now recognizes the fact that lymphatics may be formed by the 

 concrescence of multiple and independent endothelial-lined anla- 

 gen and that she has thus far presented no valid evidence that 

 these anlagen have been detached from the veins. 



Sala ('00) and more recently Miller ('13) have shown that the 

 thoracic ducts of the common fowl are formed by a concrescence 

 of independent and discontinuous lymph spaces and that their 

 endothelium is formed in situ from cells other than those which 

 constitute the endothelium of the veins. Miller has further 

 made the important discovery that groups of blood cells develop 

 in the mesenchyme along the line of the thoracic ducts and that 

 the latter subsequently convey these blood cells to the venous 

 circulation. We therefore find that hematopoiesis may actually 

 occur in connection with the development of certain lymph 

 channels, a condition which Huntington ('14) has also recently 

 verified for certain lymphatics of mammals. 



West ('14, '15), by a study of injected and uninjected embryos, 

 has recently found that the posterior lymph heart of the common 

 fowl develops in the mesenchyme and secondarily establishes a 

 connection with the veins. He has also found that hematopoi- 

 esis occurs in the mesenchyme in relation to the independent 

 anlagen of the lymph heart, and states that the blood cells thus 

 formed are not to be confounded with those which may later 

 back into the lymph heart from the veins (E. L. Clark, '12). 



Huntington ('lib), in a paper on the development of the lym- 

 phatic system in reptiles (chelonia, lacertilia), has shown that 

 the systemic lymphatics develop in the mesenchyme inde- 

 pendently of the endothelium of the veins. A particular feature 



