DEVELOPMENT OF THE THORACIC DUCT 95 



encircling vein is a venous radicle (4) almost completely sur- 

 rounded by an extraintimal lymphatic space (5) in the process 

 of replacing the atrophying vein with which it is so closely asso- 

 ciated. The same structures (4, 5) are seen still more clearly 

 defined in the corresponding position on the right side of the 

 embryo. 



Comparison with the corresponding sections just given in figures 

 103 to 109 show that the extraintimal anlage of the earlier stages, 

 developed along the identical venous radicle, has increased in the 

 13.5 mm. stage, so as to nearly envelop the vein, and that the 

 latter, if followed cephalad and caudad, is now separated from 

 the functional venous channels of this region and is in the process 

 of further recession and degeneration as the lymphatic perivenous 

 space enlarges and more and more replaces the antecedent venous 

 channel. The vein in question in the older embryo (series 189) 

 appears collapsed and shrunken, and contains only a few 1 degen- 

 erating red blood cells. In the same situation on the right side 

 of fig. 110 (series 189, slide viii, section 35), the section has cut 

 the corresponding vein and the enveloping extraintin al space at 

 right angles, so that the central kernel of the shrinking vein (4), 

 still containing a few degenerating red blood cells, is nearly com- 

 pletely surrounded by the replacing extraintimal lymphatic (5). 



Fig. Ill shows the following section of the same embryo (series 

 189, slide viii, section 36), magnified 225 diameters, in a larger 

 field which gives the topographical relations and may serve in 

 in the orientation of the succeeding plates. The ventral circum- 

 ference of the oesophagus (8) and the aorta (7) are seen dorsally. 

 The trachea (9), with the vagi (22) on either side, occupies the 

 central area. Ventrally are the two pulmonary arteries (10). 

 The section is a most important one in interpreting the early 

 histogenetic stages of the broncho-mediastinal lymphatic trunk 

 and its relation to the embryonic mediastinal venous plexus. 

 The same venous radicle (4), ventral to the vagus (22), already 

 noted in the preceeding section (fig. 110), is seen on each side. 

 On the left side it has been cut obliquely by the plane of the 

 section, and hence shows a segment of considerable length, as 

 a practically empty and partially collapsed endothelial bag (4), 



