PEVELOPMENT OF THE THORACIC DUCT 99 



traintimal lymphatic anlages of the broncho-mediastinal trunk 

 are indicated by the leader 5, while the included remnant of the 

 atrophying embryonic vein is labelled 4, as in the preceding 

 and following figures. 



Figs. 123, 124, 125 and 126 (sections 4, 5, 6 and 7) show well the 

 relation of the venule (4), undergoing atrophy and replacement 

 by the extraintimal lymphatic space (5), to the permanent 

 venous channels of the mediastinal plexus. Thus in fig. 123 the 

 dorsal large lighter area marked 4 is the blind cephalic end of a 

 venule which is nearly detached from the permanent peritracheal 

 (18} and ventral mediastinal (32) venous channels, and which in 

 the succeeding sections is seen to be partially surrounded by the 

 extraintimal lymphatic space (5) eventually designed to replace 

 it entirely. 



In figure 124 (section 5) this peri venous space (5) has developed. 

 The included vein (4) is still partially connected with the func- 

 tional peritracheal plexus (18). Further ventrad is another por- 

 tion of this same plexus. The venous kernel here still contains a 

 number of red blood-cells. These characteristic relations of 

 developing lymphatic and receding vein can be followed clearly 

 throughout the succeeding sections. The contrast between the 

 well filled permanent veins (e.g., 18, 32 in figs. 123, 124, 125 

 and 126), with walls fitted closely into the surrounding mesen- 

 chyme, and the atrophying venule surrounded by the extraintimal 

 lymphatic anlage, is striking throughout the series. Some of the 

 sections show in certain regions good pictures of nearly complete 

 reduction of the decadent vein to an insignificant central kernel 

 of connective tissue surrounded by the extraintimal lymphatic 

 space; thus, e.g., figs. 137 to 141 in the region ventro-mesad to 

 the left vagus (22). 



The 15 mm. stage shows the developing lymphatic plexus of 

 the broncho-mediastinal trunk further advanced. Figs. 142 to 

 145 show four successive transverse sections through the ventral 

 part of the upper thoracic region in a 15 mm. cat embryo (series 

 216, slide x, sections 32, 33, 34 and 35, X 225). The central 

 decadent venule in the interior of the developing perivenous 

 lymphatic space has in many places disappeared entirely, leav- 



