ORIGIN OF THE POSTERIOR LYMPH HEART 433 



mesenchymal spaces filled with colorless lymph by examining a 

 living chick under the binocular microscope. The tests just 

 described would serve to show that the blood fills the lymphatics 

 to their tips only in so far as they formed a continuous channel 

 connected with the veins, and would utterly fail to reveal any dis- 

 connected anlagen in the form of independent mesenchymal 

 spaces. The appearance of a centrifugal outgrowth of the 

 lymphatic plexus from the veins is simulated if stagnant blood 

 or any other form of injection be used as an index to the lymphatic 

 develop ;nent, for the mesenchymal spaces lying next to veins 

 are the first to make the venous connection and fill with blood 

 backed up from the general circulation. Then the spaces a 

 little more distal join the spaces already connected, in turn are 

 filled with blood, and so on until the entire blood-filled lymphatic 

 plexus is formed. Thus, while the development is proceeding 

 by the centripetal addition of disconnected anlagen, the stag- 

 nant blood in the plexus is extending in a centrifugal direction. 

 In discussing the blood-contents of the early lymphatic plexus, 

 which later forms the posterior lymph heart, the active haemo- 

 poesis in the surrounding mesenchyme is the only factor of mor- 

 phological and genetic significance. The accidental or normal 

 backing up of circulating blood into the lymphatic plexus, after 

 it has secondarily established a connection with the veins, is of 

 no significance as far as the genesis of the lymphatic structures 

 is concerned. But the in situ origin of blood cells from the 

 mesenchyme and their conveyance, via the lymphatics, into the 

 general haemal circulation is of great importance, and at once 

 places the posterior lymph hearts in the chick in the category 

 of haemophoric lymphatics, such as are met with in the thoracic 

 duct of the same form and in other vertebrates in various degrees 

 of development, as has been brought out in Huntington's paper 

 of July 1914 (10). 



Until it can be absolutely proven by some other method than 

 that of injection that all lymphatic development is centrifugal 

 growth in continuity, with invariable continuity of lumen as 

 well, such methods as this will seem to beg the question ; for the}^ 

 can afford evidence only of the degree of the centrifugal exten- 



