124 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



the original direction, following the ventral surface of the right 

 azygos. These anlages will later become confluent to form the 

 intra-thoracic portion of the right lymphatic duct (cf. 47 in fig. 

 193). 



Another set of these perivenous lymphatic spaces diverges 

 from the preceding at the level C and swings sinistro-cephalad, 

 lying in contact with the ventral surface of the interazygos anasto- 

 motic plexus and of the intersegmental arteries. 



The elements composing this series in the majority of embryos 

 of this period, (13.5 14 mm.), have already become confluent 

 to form segments of considerable length, and offer very clear pic- 

 tures of their extraintimal perivenous development, surrounding 

 radicles of the ventromedial azygos plexus. Thus figs. 206 to 213 

 show transverse sections of this embryo in the area indicated on the 

 figure of the reconstruction between the lines AA and BB. The 

 sections of the lymphatic anlage lying ventral to the right inter- 

 segmental artery in this region show the central decadent venule 

 and the enveloping extraintimal lymphatic space almost diagrama- 

 tically. 



Fig. 189 is taken from a similar reconstruction of a 14 mm. cat 

 embryo (series 34 of the Princeton collection) . 



The right (3'} and left (6'} azygos veins arch ventrad to their 

 point of confluence with the respective precavae (3, 6}. 



The interazygos anastomosis forms a dense closely woven supra- 

 aortic plexus, perforated by the intersegmental arteries. In gen- 

 eral the lymphatic development has not quite reached the stage 

 represented by the preceding series (76), especially along the 

 main right and left azygos trunks, where the anlages are still 

 relatively few and of small extent. On the other hand, the lym- 

 phatic line which in series 76 (fig. 188) is seen to swing mesad to 

 the ventral surface of the interazygos anastomosis at the level of 

 C and thence continues to ascend sinistro-cephalad, is already 

 well developed in series 34 (fig. 189). The level C of the preced- 

 ing figure (188) corresponds in fig. 189 to the upper of the two 

 lines A-B. Cephalad of this level a well defined chain of 

 lymphatic anlages extends, diverging to the left from the main 

 right azygos line, along the ventral aspect of the interazygos 



