70 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



may be played by the veins in establishing such communi- 

 cations with the independently formed lymphatics? In the 

 development of the general vascular system which includes 

 the arteries, veins and lymphatics, the end result desired is the 

 formation of a connected system of vessels which subserve 

 definite functions in the economy of the general vascular system. 

 If the general vascular system develops progressively in a uni- 

 form manner, by a concrescence of independently formed anlagen, 

 and the lymphatics, as is the case, form the last link in complet- 

 ing the chain, it is evident that the same factors should account 

 for the establishment of a connection between the veins and the 

 independently formed lymphatics, as between the independently 

 formed anlagen of which the veins and lymphatics are originally 

 composed. Such a connection could alone be established, either 

 by a growth or sprouting of the endothelium of the veins; by 

 means of a further in situ differentiation into endothelium of the 

 mesenchymal cells which intervene between the veins and the 

 independent anlagen of the lymphatics; or, both of these factors 

 might be involved. If a connection should be established by 

 a sprouting of the endothelium of the veins, such sprouts would 

 possess no significance beyond the fact which we all recognize, 

 that all vascular endothelium is capable of growth after it has 

 once been formed. The question, however, is not whether 

 endothelium is capable of growth, but rather what are its limita- 

 tions and how is the endothelium formed that does the growing. 

 It is therefore evident, if, at the points at which the lymphatics 

 establish permanent communications with the veins, venous 

 endothelium should contribute to the formation of the lymphatics, 

 it would play only a subsidiary role, and serve only as a means, 

 in common with the endothelium of other independently formed 

 vascular anlagen, of bringing two independently formed portions 

 of the vascular system into communication with each other to 

 form a continuous system of channels. Such veno-lymphatic 

 connections have been hitherto described by Huntington and 

 McClure ('10) under the name of Venolymphatics.' It is evi- 

 dent, however, that these 'venolymphatics' would not differ, 

 in any sense, from other connections, where a similar growth of 



