66 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



embryo of the left supracardinal (left azygos) and left postcardi- 

 nal veins and the left duct of Cuvier. 



It is evident and appears clearly in our earlier publications, 

 that the fundamental plan of development followed by these 

 replacing lymph channels does not depart from that followed by 

 other lymph channels, either in mammals or in any other verte- 

 brates where the development of the lymphatics is unaccompanied 

 by the replacement of degenerating veins. Where, as in the 

 case of the trout, lymph channels do not develop along the course 

 of degenerating veins, an extraintimal replacement of a de- 

 generating vein by a lymphatic necessarily does not occur. 

 It is therefore plain that the extraintimal replacement, as described 

 by us, possesses only a mechajiical significance, and is 7nerely an 

 adaptatio7i of a common plan of lymphatic genesis, through the 

 concresence of independent an^agen, to the local conditio7is lohich 

 prevail only in certain districts of the mammalian eynbryo. 



The same general plan of development as outlined above by 

 Huntington and McClure for the Ij^mphatic system of the cat 

 has also been found by Kampmeier ('12a, '12b) to occur in the 

 embryo of the pig. His description of the independent and dis- 

 continuous anlagen of the thoracic ducts which he found in the 

 injected pig embryo loaned him by Professor Sabin, needs no 

 further comment (JMcClure '12). 



F. T. Lewis ('05) has described the presence of a chain of dis- 

 continuous 'Ij^mphatic spaces' (endothelial-lined anlagen) in 

 the rabbit embryo which lie along the azygos veins in the path 

 of the future thoracic duct. He regards these anlagen, however, 

 as having been detached from the veins. Concerning these 

 multiple anlagen of Lewis, Sabin ('08) has stated as follows: 



Since these spaces are lined with a definite endothelium, they form 

 a much more serious obstacle to the theory of growth of the lymphatics 

 from the endothelium of the veins than the more indefinite spaces 

 to be found in earlier embrj^os, and I cannot but think that if these 

 multiple endothelial-lined isolated spaces do exist along the veins in 

 later stages, they would form serious evidence against the theorj^ 

 of the origin of the lymphatics from the veins. Or at least if the 

 lymphatics, in their growth, do pick up isolated endothelial-lined 



