130 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



to lose their connection with the azygos trunks, and appear as 

 partially detached venous elements closely applied to the ventro- 

 medial wall of the main vein. 



Thus the ventro-medial tributary 34 in fig. 202, (slide x, sec- 

 tion 30), still in open communication with the main right azygos 

 vein (3), is seeninthe next section (fig. 201, slide x, section 31) to be 

 separated from the same, and can be followed for a considerable 

 distance caudad, closely applied to the ventro-medial aspect of 

 the parent trunk, but no longer opening into the same or con- 

 nected with adjacent elements of the azygos plexus. A venule of 

 this type will, in the succeeding stages, form the line along and 

 around which the extraintimal anlages of the thoracic duct will 

 develop. 



Two successive transverse sections of the central thoracic 

 region in a 13.5 mm. embryo (series 189, ^lide x, sections 15 and 

 16, X 225) are shown in figs. 198 and 199. The ventro-medial 

 azygos venous plexus (#4) is still prominent, especially on the right 

 side, between right azygos vein (3) and aorta (7). Some of its 

 ventral and lateral components (4) are, however, detached from 

 the main plexus and are beginning to be surrounded by the early 

 extraintimal anlages of the thoracic duct (<5). 



In another embryo of this period (series 76), extraintimal lym- 

 phatic development in the axygos region is further advanced than 

 is usually the case in the 13.5 mm. stage, i igs. 206 to 213 inclu- 

 sive show transverse sections through the middle thoracic region 

 of this embryo (series 76, slide xi, sections 15, 16, 17, 19, 21, 22, 

 23, 28, X 300). The microphotographs are to be referred to the 

 ventral view of the reconstruction of slides x, x', xi , xiii, 

 and part of side xiv, (155 15 n sections) shown in fig. 188 and 

 are included within the area bounded in this figure by the lines 

 A-B. They show especially the development of the prominent 

 extraintimal intermediate segment of the thoracic duct an!age 

 on the ventral aspect of the interazygos plexus and of the fourth 

 intercostal artery of the right side. Fig. 206 (section 15 of slide 

 xi) shows the right azygos vein (3), the aorta (7) and the well 

 developed interazygos plexus (15). A radicle of the latter (4), 

 in the interval between the structures named, is almost completely 



