GENERAL CONSIDERATIONS 69 



Allen ('13) has investigated the development of the lymphatic 

 system in Polistotrema (Bdellostoma) stouti and speaks of the 

 lymphatics of fishes as veno-lymphatics. He states: ''I expect 

 to show that the main factor in the construction of the veno- 

 lymphatic system is the same as was described for the caudal 

 lymph hearts, namely, the formation and union of certain 

 mesenchymal spaces." 



Allen, independently of Miller, has also observed that an active 

 hematopoiesis occurs in the mesenchyme in relation to the de- 

 veloping caudal lymph hearts of Polistotrema. 



Except for differences of opinion regarding the origin of lym- 

 phatic endothelium, it may be observed that the above-mentioned 

 investigators agree for the most part that the continuous lumina 

 of the lymphatics, like those of the haemal vessels, are formed 

 by a concrescence of independent and discontinuous anlagen. 



If we disregard entirely the personal equation which may have 

 influenced any or all of the above-mentioned investigators to 

 interpret their findings in accordance with one or the other view, 

 the fact still remains that the anlagen of the lymphatic system, 

 as observed in sections of injected and uninjected embryos, 

 have been found to be identical with those of the intra-embryonic 

 blood-vascular system, which has been shown by sections and 

 experiment to be formed by a concrescence of independent 

 anlagen, and its endothelium to be formed in situ from mesen- 

 chymal cells. Since we possess exactly the same kind of evidence 

 in favor of the mesenchymal origin of lymphatic endothelium, as 

 we formerly did for that of the intra-embryonic hae7nal channels, 

 before experiment was applied, it therefore seems highly improbable 

 that the endothelium of two sets of similarly appearing anlagen, 

 belonging to the same general organ system and developing side by 

 side, should differ in its mode of origin, rather than follow a common 

 gejietic plan. 



We know that the lymphatics, both in the embryo and in 

 the adult, establish a permanent communication with the 

 veins at typical points (McClure and Silvester '09, and Sil- 

 vester '12). The question therefore arises, what role, if any. 



