RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 19 



beginning independent of the haemal endothelium, and develops 

 with the first appearance of the lymphatic spaces, from the 

 indifferent mesodermal cells lining these spaces. In my opinion 

 the lymphatic and haemal vascular endothelium have the same ge- 

 netic derivation from the modified mesodermal cell lining the tissue 

 spaces. The primary stage of endothelial differentiation is the 

 same, whether the resulting channel system is to be assigned to 

 the haemal or the lymphatic division of the vascular system. 

 We have therefore two generations of the embryonic vascular 

 endothelial cell, a haemal and a lymphatic. Both develop in 

 the same way and as the result of identical genetic factors from 

 the indifferent mesodermal cell. Both are from the very begin- 

 ning of the process independent of each other in the mammalian 

 embryo. 



I desire again to emphatically aver my conviction that all the 

 systemic lymphatic vessels of the mammalian embryo, includ- 

 ing the thoracic ducts and their tributaries, are neither in their 

 genesis continuous " outgrowths" or "buds" from sacs of venous 

 origin, wherever situated, nor derived from multiple outgrowths 

 from the embryonic veins, such outgrowths subsequently separat- 

 ing from'the veins and fusing into continuous lymphatic channels. 

 They are, in my opinion, on the contrary, from their very first 

 inception, independent of the haemal vascular system, and their 

 endothelial lining is not derived from the blood vascular endo- 

 thelium. They develop as independent intercellular mesodermal 

 spaces, which become confluent with each other to form larger 

 and larger communicating channels. These finally attain their 

 entrance into the venous system through the intervention of 

 the jugular lymph sacs, in the manner outlined in the publica- 

 tions above quoted. 22 > 26 > 27 



Before taking up the details of the development of the first 

 lymphatic anlages in the mammalian embryo, it seems advisable 

 to refer briefly to a resume of known facts in regard to the earliest 

 formative stages of the blood vascular system, in order to facili- 

 tate the comparison between haemal and lymphatic development. 



Phylogenetically, the earliest form of a closed circulatory 

 system in multicellular organisms consists of intercellular canals 



