DEVELOPMENT OF THE THORACIC DUCT 103 



results of a careful examination of lymphatic development along 

 the lines here indicated. 



No competent and impartial observer can mistake the signifi- 

 cance of the conditions here shown. Every stage of the process 

 can be followed in detail. The behavior of the decadent embry- 

 onic vein, and its relation to the enveloping extraintimal lymphatic 

 channel, are absolutely demonstrated. The endothelium of the 

 shrinking vein has no share in furnishing the independent lym- 

 phatic endothelium of the replacing mesenchymal space, and 

 nowhere, in the entire process, is there the faintest suggestion 

 of an " out-bud" or of a "splitting off" from the circumference of 

 an otherwise valid embryonic vein of "lymphatic" or "veno- 

 lymphatic" anlages. 



It is evident in comparing a number of embryos between 13 

 and 16 mm., crown-rump measure, that the extensive area occu- 

 pied by the lymphatic anlages in the earlier stages (13 mm., 13.5 

 mm. and 14 mm.) is due to the relatively large size of the central 

 decadent vein. In the earlier stages (e.g., figs. Ill, 112, 113, 114, 

 119, 121, 125, 126, 127, series 189, 212 and 214) the evacu- 

 ated and abandoned vein appears as a wide loose and partially 

 collapsed endothelial bag, surrounded by a narrow extraintimal 

 lymphatic space, with but as yet indifferently defined endothelial 

 lining, continuous at one or more points with the external surface 

 of the endothelium of the degenerating vein. As development 

 proceeds the actual perivenous lymphatic spaces increase but they 

 concentrate and narrow down on the contained venous kernel, 

 and, as the latter continues to decrease in size and finally become 

 completely eliminated, the resulting clear lymphatic channel oc- 

 cupies relatively less space than that filled in the earlier stages 

 by the decadent venule and the early surrounding lymphatic 

 anlage combined. 



Thus in the 15, 15.5 and in some 16 mm. embryos (figs. 142, 

 143, 144, 145, 146, and 147, series 216, 215 and 230), while the 

 actual lumen of the lymphatic channel has increased that of the 

 enclosed and abandoned vein has correspondingly receded to an 

 insignificant remnant, which projects into the lymphatic space 

 from a narrow pedicle attached to the latter's endothelial lining. 



