136 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



ture on the right, begins to make its appearance in slide xv of 

 this embryo (series 214, section 33) in relation to a rudimentary 

 ventral tributary of the left azygos vein (section 32) . It can be 

 followed through the rest of the slide to section 36, and in the 

 succeeding slide xvi to section 7. It reappears in section 15 of 

 slide xvi, again in association with a reduced ventral azygos 

 branch, and continues to section 26, where it seems to end. It 

 reappears in the following section (27) again as the extraintimal 

 envelope of a small retrograding ventral azygos branch, and in- 

 creases rapidly in size, ending in section 30. It again appears in 

 section 33, and can be traced with frequent interruptions from this 

 point caudad, diminishing in size and continuity, while conversely 

 the ventral azygos tributaries of this area are larger and better 

 developed (cf. left side of reconstruction, fig. 190). 



Certain 14 mm. embryos, in which the development of the thor- 

 acic duct anlages in the middle and lower thoracic region has not 

 yet reached the condition usually encountered in average embryos 

 of this period, furnish excellent examples of the early appearance 

 of the first lymphatic spaces in their relation to the components 

 of the ventro medial azygos tributary plexus, which they are des- 

 tined to eventually replace. 



Thus, in another 14 mm. embryo (series 127, slide x, sections 

 3, 4, and 10, X 225), shown in transverse section in figs. 241, 

 242, and 243, the ventro-medial azygos plexus (34 in fig. 241) is 

 still well preserved and in open communication with the main right 

 azygos trunk. The following section (fig. 242) shows the same 

 conditions, but a small extraintimal lymphatic space (5) appears 

 in relation to one of the component venules (4) of the peleus, 

 applied to its dorso-lateral circumference. The same beginning 

 of extraintimal lymphatic space development in relation to azygos 

 tributaries is seen further caudad in the same slide of this embryo 

 (figs. 243, series 127, slide x, section 10). 



One of the best examples of the early stages of development of 

 the thoracic duct anlages in relation to the ventro-medial tribu- 

 taries of the azygos veins is afforded by a 14 mm. embryo (series 

 34 of the Princeton University embryological collection). I owe 

 the opportunity of examining and reconstructing this remarkably 



