RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 53 



In view of the fact that these first anlages can be traced step 

 by step as developing into the systemic lympnatic channels of the 

 adult, it becomes apparent that their origin cannot be ascribed 

 to " outgrowths" from the jugular sacs, as yet not formed. A 

 similar lack of correct reasoning appears in portions of the recent 

 anatomical literature in reference to the injection of lymphatics 

 in the early stages. It is manifestly impossible to inject some- 

 thing which does not as yet exist in the form of a continous and 

 injectable system of communicating channels. The spaces which 

 I am describing and which I regard as the first anlages of the 

 systemic lymphatics cannot be injected at the time of their first 

 appearance because they are intercellular spaces, extremely nu- 

 merous, but as yet isolated from each other, and not in communi- 

 cation with veins or with anything else that could be injected, 

 in the sense of filling a lumen artificially with a colored fluid, or 

 air, or any other foreign substance. They multiply rapidly, 

 become confluent to form larger spaces, and these finally unite 

 into continuous channels. But the genetic determination of the 

 origin of the lymphatic system must be based on the close study 

 of embryonic stages long prior to the formation of continuous 

 channels capable of injection. Where lymphatic vessels can once 

 be demonstrated by this method they have passed the formative 

 stage. It then becomes merely a question of topographical ex- 

 tension and readjustment, such as is encountered in all vascular 

 structures alike in the later embryonic periods. 



The real problem of the genesis of the lymphatics can only be 

 solved by the recognition of tne earliest formative stages, begin- 

 ning with the first appearance of the mesodermal spaces above 

 described. 



Finally I can sum up my observations, and the views thereon 

 based, as follows: 



1 The systemic lymphatic vessels of the entire body arise, in 

 the mammalian embryo, by confluence of mesodermal spaces, 

 developed on the same lines as those governing the formation of 

 the earliest haemal vascular channels, but independent of the 

 same. 



