ORIGIN OF THE POSTERIOR LYMPH HEART 407 



finally lumina, which could not be injected from the lymph 

 heart, appeared. The lumen of each strand he found to be 

 lined with endothelial cells. By the continuation of the space 

 formation lymphatic vessels developed which connected second- 

 arily with the similarly acquired lumina of other cell strands 

 which had appeared within the heart. It was not until this 

 connection was established that it was possible to inject the 

 lymphatic vessels from the heart. 



Allen (8) in a recent important publication on Polistotrema 

 (Bdellostoma) describes the caudal lymph heart as arising from 

 isolated mesenchymal spaces in the region of the anterior end 

 of the two branches of the caudal vein, and the ultimate fusion 

 of these spaces by the breaking down of their partitions. Inci- 

 dent to this process certain cells in the interior of the system 

 of spaces become spherical and are transformed into red blood 

 corpuscles. Secondarily the cavity of the lymph heart estab- 

 lishes connections with the caudal vein by the same process, 

 that is a breaking down of mesenchymal partitions while periph- 

 erally the cavity is enlarged by the new formation of isolated 

 mesenchymal spaces and their ultimate annexation. C'oinci- 

 dentally the mesenchymal cells bordering the cavity of the 

 lymph heart flatten to form its endothelium. Allen in con- 

 clusion says that his 



* * * * studies thus far indicate that the most primitive form of 

 lymphatic system are veins that function for both lymphatics and 

 veins. Hence it would be expected that ontogeny would repeat the 

 phylogeny of the lymphatics, and instead of having their origin directly 

 from the veins, they would begin directly as the veins did, by the vacuo- 

 lization of the original mesenchyme. 



T'hese vessels Allen has designated 'veno-lymphatics.' The 

 recognition of haemopoesis in the vicinity of developing lym- 

 phatics and from their endothelium is of major morphological 

 importance and the substantial agreement between the results 

 of Allen and of Miller should go far to clear up some of the 

 difliculties that have beset the study of the ontogeny of the 

 lymphatic system. The term veno-lymphatic was used by 

 Huntington and McClure (9) in their studies of the mammalian 



