GENERAL CONSIDERATIONS 65 



lymphatics, like the haemal vessels, are formed by a concres- 

 cence of independent and discontinuous anlagen, and that their 

 endothelium arises t in situ from intra-embryonic mesenchymal 

 cells. 



A principal contention of Huntington and McClure ('07a) 

 regarding the development of the lymphatic system has been 

 that its anlagen arise independently and discontinuously in the 

 embryo, and that its endothelium does not spread continuously 

 and uninterruptedly throughout the body from the endothelium 

 of the veins. We have repeatedly shown that the lumina of the 

 lymphatics are formed by a concrescence of discontinuous and 

 independent lymph vesicles or lymph spaces, and that 'the cells 

 which constitute the walls of these spaces are derived in situ 

 from mesenchyme and not from the endothelium of the veins. 

 In the early stages of our investigations we laid especial stress 

 upon a plan of development for the lymphatic system of mammals 

 which we described under the name of the 'extraintimaF theory 

 of lymphatic development, and which may be briefly described 

 as follows : The development of the thoracic ducts (Huntington, 

 '10, 'lla) and mesenteric lymphatics (McClure, '10) in the cat 

 is correlated with the degeneration of certain venous channels, 

 many of which are tributaries of the azygos division of the 

 supracardinal veins (Huntington and McClure, '07b). A series 

 of independent lymph spaces arise discontinuously in the 

 mesenchyme external to the intimal lining of these degenerat- 

 ing vessels and, as these lymph spaces gradually become con- 

 crescent to form continuous channels, the latter, following a 

 line of least resistance, utilize the static line vacated by these 

 degenerating veins. In this manner certain of the main lymph 

 channels of the mammalian embryo follow the course of and 

 finally occupy completely the territory formerly occupied by veins. 

 This principle of extraintimal replacement of certain evanescent 

 embryonic venous channels by lymphatics accounts for the sin- 

 istral drainage plan finally assumed by the thoracic duct system 

 in the embryo of the cat. The cranial or azygos division of 

 the left thoracic duct of the embryo persists as the main line of 

 drainage in the adult, in correlation with a degeneration in the 



