DO PERIPHERAL LYMPHATICS FORM A CLOSE SYSTEM 55 



that a series of independent lymph vesicles intervene between 

 these two primary drainage centers and that they lie in the 

 course subsequently followed by the lateral pharyngeal lym- 

 phatic. It has also been shown that the subocular lymph sacs 

 cannot be injected from the veins until this intervening portion 

 of the lateral pharyngeal lymphatic is completed and until the 

 latter has established a connection with the subocular lymph sac. 

 Such being the case, it is evident that the pronounced varia- 

 bility observed on the nineteenth, twentieth and twenty-first 

 days, must be related to the difference in the rate at which a 

 connection has been established between the primary posterior 

 drainage center of the pharynx, and the independent anlagen 

 (independent lymph vesicles) of the lateral pharyngeal lymphatic 

 which intervene between the primary posterior drainage system 

 of. the pharynx and the subocular lymph sac. 



XII. DO THE PERIPHERAL LYMPHATICS FORM A CLOSED SYSTEM 



OF ENDOTHELIAL-LINED VESSELS SIMILAR TO THOSE 



OF THE HAEMAL SYSTEM? 



The presence of a closed system of peripheral Ij^mphatics is 

 generally regarded as a necessary corollary to the theory that the 

 lymphatics sprout centrifugally and continuously throughout the 

 body of the embryo fiom the endothelium of the veins.' On the 

 other hand, it is generally inferred that the view which calls for a 

 local origin of lymphatic endothelium from tissues other than 

 from a preexisting venous endothelium, demands the presence of a 

 permanent open communication between the peripheral lym- 

 phatics and the intercellular tissue spaces. I may state that the 

 falsity of this inference becomes apparent when we consider that 

 recent investigations have conclusively shown that the endothe- 

 lium of the entire blood-vascular system of vertebrates — which 

 is a closed system — possesses a local origin. It is therefore evi- 

 dent that the view which calls for a local in situ origin of lym- 

 phatic endothelium, and for the development of lymphatic chan- 

 nels through the confluence of independent lymph vesicles, does 

 not necessarily preclude the possibility that a closed system of 



