458 OTTO F. KAMPMEIER 



earliest anlagen of the blood vessels arise between the mesodermal 

 cell-strands as isolated spaces and fissures, which at first are 

 bounded by ordinary cuboidal cells but later acquire the charac- 

 teristic vascular endothelium by the modification of these cells. 

 Being plastic, the cells lining either a haemal or a lymphatic 

 anlage must be regarded as obeying the internal pressure of the 

 cavity and becoming more and more flattened and endothelial- 

 like as the pressure of the fluid or plasma increases. 



Although this work deals primarily with the source of the thor- 

 acic duct, attention was not confined to it exclusively but also 

 considered briefly two other lymph ducts, the mediastinal chan- 

 nel draining the mediastinum and its organs, and the right lym- 

 phatic duct, which in earlier stages of phylogenesis undoubtedly 

 composed a part of the thoracic duct system but now ordinarily 

 remains independent of it and receives tributaries only from the 

 cephalic, cervical and upper thoracic regions of the right side. 

 In development these channels repeat in all details the history of 

 the thoracic duct, arising as isolated mesenchymal or perivenous 

 spaces which subsequently become confluent. 



After the above description of the development of the thoracic 

 duct and a consideration of the evidence presented, attention 

 may be directed towards several criticisms advanced recently by 

 those investigators opposed to the view of the direct mesenchymal 

 origin of lymphatic vessels. These opponents would dismiss as 

 artifacts all of the 'lymphatic anlagen' described by the writer. 

 Sabin, referring to the figures of extra-intimal replacement dis- 

 covered by Huntington and McClure in their investigations on the 

 genesis of the lymphatic system in the cat, maintains that "they 

 are all in the center of the embryo where the fixing fluid pene- 



chick embryo, brought forth conclusive evidence that the haemal channel system 

 of ths extra-embryonic area is developed from isolated spaces, which arise blindly 

 in the undifferentiated mesenchyme and which subsequently change their shape 

 by expansion and elongation and become confluent with other such spaces to pro- 

 duce the complicated blood plexuses in the area designated. The "spaces are 

 frequently bounded by a mere line, more or less retractile in character. In others 

 the lumen is lined with rounded or oval cells which later become fusiform and 

 flattened. 



