SUMMARY AND CONCLUSIONS 157 



Observations made on vascular structure and growth in the 

 transparent tails of living anure amphibia have of late been 

 offered in support of the theory of continuous centrifugal lym- 

 phatic development from pre-existing venous endothelium. These 

 observations recall vividly the corresponding historical period 

 in the developmental analysis of the embryonic blood-vascular 

 channels, prior to Thoma's classical researches on this subject. 

 Sigmund Mayer nearly thirty years ago employed the tails of 

 living anure larvae for the purpose of studying this phase of the 

 circulatory system. It seems strange that to-day it should be 

 equally, or even more, difficult to induce observers to differentiate 

 between development and growth of vascular channels. Develop- 

 ment means the genesis of the first embryonic anlages of a vascular 

 system. When this has once been established its further exten- 

 sion by growth is an entirely different problem which demands 

 equally careful and competent observation and presents many 

 phases difficult of interpretation, involving, among others, the 

 entire question of the 'specificity of vascular endothelium.' 

 Much futile and controversial discussion could be avoided if 

 the growing generation of contributors to this subject will re- 

 strain their enthusiasm long enough to bear this vital difference 

 in mind. 



On the other hand, the theory which regards the lymphatic 

 channels of the mammalian embryo as developing by the coa- 

 lescence of multiple venous 'outgrowths' or portions of embryonic 

 venous plexuses, detached and separated secondarily from the 

 parent veins, deserves detailed consideration. 



According to the postulates of this view, the lymphatic endo- 

 thelium is the direct descendant of the pre-existing haemal vascu- 

 lar endothelium lining the detached venous elements which enter 

 into the formation of the lymphatic channels. The latter, after 

 being thus established, make secondary and permanent connec- 

 tions with the venous system. 



It is of course a very easy matter to prove in successive stages 

 the topographical correspondence of venous and lymphatic chan- 

 nels, to prove, in other words, that the area occupied by a vein in 

 the earlier stages is filled in the following periods by a lymphatic 



