54 Florence R. Sabin. 



veins, though he confuses the picture by considering them as coming 

 from tissue spaces at the same time, describes the thoracic duct as 

 coming from solid cords of mesenchyme, and the peripheral vessels 

 as derived from spaces in the mesenchyme. We find nothing to 

 correspond to the solid cords of mesenchyme as an anlage of the 

 thoracic duct in mammals, and believe that the lymph vessels grow 

 out from the primitive sacs. That is, we believe that the conditions 

 found by H. Hoyer for amphibia, that the vessels come from the 

 hearts, is true also in mammals. This being the disputed point 

 however, it will be necessary to review the literature in this connec- 

 tion with care. 



In 1901 I showed that the jugular lymph sacs are the primary 

 lymphatics in mammals, that they are derived from the veins, that 

 from these sacs, and others, vessels grow out to invade the body and 

 that therefore there are non-lymphatic areas and one can study the 

 invasion of these areas by lymphatic vessels. In the study of the 

 skin this general law was found to hold, that there are areas which 

 at first cannot be injected either directly or through the sacs. This 

 I believe to be because there are no lymphatics to inject. That 

 gradually lymphatics invade these areas and at first a primary 

 subcutaneous plexus can be injected, later a secondary more super- 

 ficial plexus, and finally terminal capillaries in the papillae. The 

 same law holds for the lymphatics of the intestine as shown by Dr. G. 

 Heuer in the same number of this journal. In the intestine the 

 lymphatics first form a plexus in the submucosa; secondarily a 

 mucosal capillary plexus forms and from this mucosal plexus the 

 lacteals grow out. In connection with the intestine the fundamental 

 point that the lymphatics grow out from the sacs is also shown. 

 In all the early work the injections of the intestine were made 

 through the thoracic duct, but later it proved that by far the best 

 place to inject is the retroperitoneal sac. This sac gives the key for 

 working out the development of the lymphatics of the viscera. For 

 years I have been trying to get injections of the lymphatics of the 

 lungs and diaphragm and have never succeeded until I intro- 

 duced the needle directly into the retroperitoneal sac. In connec- 

 tion with the intestine, injections into the retroperitoneal sac at 



