154 SYSTEMIC LYMPHATIC VESSELS IN DOMESTIC CAT 



It is quite true that it is possible to inject the jugular lymph 

 sacs in certain mammalian embryonic stages and thus, by grad- 

 ing the preparations, to obtain pictures which might be interpreted 

 in the sense of this theory, provided all questions as to the onto- 

 genetic history of lymphatic organization are designedly left out 

 of consideration. There may be a royal road to the solution of the 

 vertebrate lymphatic problem, but surely the injection method 

 has not found it. It will furnish abundant illustrations of a pre- 

 sumably progressive extension of lymphatic development from 

 centre to periphery, but no proof of the actual occurrence of the 

 process. The tissues of the developing vascular system in mam- 

 malian embryos afford at no period and in no region the slightest 

 evidence of such an occurrence. On the other hand, the mammal- 

 ian embryo does offer conclusive evidence of the development of 

 mesenchymal spaces, arising independently of the veins, and lined 

 by a lymphatic endothelium which is not derived from a pre-exist- 

 ing haemal endothelium. These spaces form the anlages of the 

 future systemic lymphatic channels, and they are continuous with 

 intercellular tissue spaces limited by the surrounding indifferent 

 mesodermal cells which have not yet assumed endothelial charac- 

 ter. This relation of lymphatic anlages, lined by lymphatic endo- 

 thelium, to the intercellular mesenchymal tissue spaces, while 

 clearly determined in mammalian embryos, is moreover strikingly 

 and diagrammatically demonstrated in reptilian embrj^os. 



I have given in the preceding pages my reasons for assigning 

 an absolutely negative value to injection experiments in de- 

 termining lymphatic ontogeny. These preparations yield interest- 

 ing topographical pictures in the different stages, showing the 

 extent of continuous lymphatic organization at different periods, 

 but they do not touch the problem of the genetic processes respon- 

 sible for the establishment of a set of vascular channels capable 

 of injection. Much of the general confusion of ideas evident in 

 discussions of the question arises, in my opinion, from the failure 

 to correctly appreciate the value and significance of the lymph- 

 heart remnants of venous origin which in the mammal are interpo- 

 lated as junctional links between the independently developed 

 systemic lymphatic vessels and the permanent functional veins. 



