444 OTTO F. KAMPMEIER 



In series 192 a 21.5 mm. embryo slightly older than the preced- 

 ing embryo 23a, the right thoracic duct anlage (5d) extends as a 

 continuous channel back to the point X, as indicated on figure 35. 

 On comparing this with figure 30, it can be observed that the 

 longer portion of the postcardinal division of the right anlage, 

 which in Sabin's series 23a exists in the form of the long fusiform 

 space described above, has established connection with the pre- 

 cardinal thoracic duct segment, and in this way it has increased 

 considerably the length of the channel joined to the jugular lymph 

 sac. Like the fusiform space of that embryo, the postcardinal 

 thoracic duct division of series 192 is irregularly beaded or vari- 

 cose, the constrictions or nodes suggesting more recent fusion 

 between successive internodes. That this suggestion is a fair 

 one is substantiated by the fact that toward the region of the peri- 

 aortic veno-lymphatics, or beyond the point X (fig. 35) where the 

 continuous anlage ends blindly, it is followed at intervals by a few 

 large mesenchymal vacuoles (Jf.d) between which no communica- 

 tion is as yet noticeable save through the indifferent tissue net- 

 work. 



The continuous portion of the left thoracic duct anlage in series 

 192 is perhaps no longer than in Sabin's series 23a, but the blind 

 lymphatic spaces (^s, fig. 35) following it are far more extensive 

 in length, especially those in the region of the left Cuvierian duct. 

 Here there are two long spindle-shaped spaces parallel to each 

 other, the shorter one of them being that portion of the anlage 

 of the future mediastinal lymphatic vessel situated near the point 

 of its subsequent junction with the other, or longer space, which 

 represents an anlage of the left thoracic duct. In a 22 mm. 

 embryo, a slightly older stage, all of the blind spaces of the pre- 

 and post-cardinal divisions have become confluent to form the 

 uninterruped duct and its mediastinal tributary. 



(C) Supracardinal division. In the third division of the thor- 

 acic duct area there may be recognized an anterior and a posterior 

 half, those regions, respectively, in which the duct-anlage during 

 its initial development is associated with the periaortic (6c, fig. 

 3) and the posterior supracardinal [6c, fig. 4) veno-lymphatics. 



