138 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



(r5) is larger and still contains a number of red blood cells. In the 

 four succeeding sections, 19, 20, 21 and 22 (figs. 248, 249, 250 and 

 251) the former line of communication of the decadent venule (4) 

 with the functional azygos channel can still be distinctly traced as 

 a strand of connective tissue cells joining it to the beginning of the 

 right interazygos plexus. The separation of the venous radicle 

 undergoing extraintimal lymphatic replacement has just been 

 accomplished in this area. 



The six sections of this embryo shown in figs. 246 to 251 should 

 be compared with the slightly more advanced 14 mm. embryo 214; 

 where, e.g., in figs. 230, 231, 232, 214, 215, 216, 217, 218 and 219 

 exactly equivalent sections are given, showing the next step in ex- 

 traintimal lymphatic development in this region. The atrophying 

 venule (4) in the earlier embryo (series 34) still contains red 

 blood cells and still betrays its original connection with the func- 

 tional venous channels. In the genetically slightly older embryo 

 (series 214) the same structure appears now as an empty endothe- 

 lial bag (4), completely detached from the azygos veins. In both 

 embryos this decadent venous radicle is surrounded by the de- 

 veloping extraintimal lymphatic space (5}. The comparison of 

 the sections shows every step in the process of development in 

 these two closely connected stages. The reduced photographic 

 reproductions do not give the clear and unmistakable pictures 

 presented by the actual sections, owing to the loss of the differ- 

 ential stain and of the focal adjustment. Still the contour of the 

 venous core, undergoing atrophy and replacement by the extrain- 

 timal lymphatic anlage, its content of red blood cells and its 

 recently interrupted connection with the main azygos vein in the 

 younger embryo (series 34), can be followed with a sufficient degree 

 of clearness and can be contrasted with the conditions presented 

 by the succeeding stage (series 214). 



Figs. 244 and 245 give two successive sections from the caudal 

 part of the thoracic region in series 34 (slide xxxi, sections 18 

 and 19, X 225) in which the earlier conditions, preceding the first 

 appearance of the lymphatic anlages of the thoracic duct in the 

 azygos segment, are shown. 



Between the right azygos vein (3} and the aorta (7) are seen 



