DEVELOPMENT OF THE THORACIC DUCT 101 



which have become fully established. Their lumen now appears 

 for the most part entirely cleared of the remnants of the decadent 

 venules originally contained therein. In only a few instances 

 does the original vein kernel appear reduced to an insignificant 

 rudiment. The resulting plexus of the broncho-mediastinal lym- 

 phatic duct fills the typical position occupied in the earlier stages 

 by the extensive preceding perivenous lymphatic reticulum. 

 Thus figs. 148-153 give transverse sections through the ventral 

 part of the upper thoracic region in another 16 mm. cat embryo 

 (series 222, slide X, sections 1, 2, 3, 10, 15 and 16, X 225). This 

 embryo affords remarkably clear pictures of the fully devel- 

 oped lymphatic plexus of the ventral or broncho-mediastinal 

 trunk. 



Figs. 148 and 153 (sections 1 and 16) give the topographical 

 pictures of the entire region involved. The broncho-mediastinal 

 lymphatic plexus (37) is fully developed in the typical area between 

 the trachea (8) and vagi (22) dorsally and the pulmonary arteries 

 (10) ventrally. The lymphatic spaces possess a clear lumen and 

 distinct endothelial walls. The remnants of the degenerating 

 ventral mediastinal venous plexus which they have replaced have 

 almost entirely disappeared. Only slight traces of them are 

 still to be seen here and there. In the main the lymphatic chan- 

 nels are clear of venous kernels and form a rich interlacing lym- 

 phatic plexus. The central field contains the oesophagus (8), 

 aorta (7) and part of the medial circumference of the left praecava 

 (6). Dorsally on the right side the right azygos vein (3) and 

 sympathetic nerve (1) appear, while on the left side the section 

 includes the cephalic part of the azygos segment of the left thoracic 

 duct (36). 



Sections 2, 3, 10 and 15 of the same slide are shown in their 

 ventral portions in figs. 149 to 152. They all exhibit the extent 

 and continuity of the broncho-mediastinal lymphatic channel 

 complex in this stage. The figures are not leadered, but the 

 individual structures can be readily identified by reference to 

 the topographical figures 148 and 153. 



The succeeding stages are characterized by a condensation of 

 the extensive lymphatic plexus of the typical 16 mm. and 17 mm. 



