210 FREDERICK TILNEY 



venous system. The venous orifice of this duct-like structure 

 is wide; the saccular orifice is elongated and narrow, while the 

 duct itself is placed between the walls of the sac and the vein. 

 This arrangement makes it appear that distension of the vein 

 would act on the jugulo-subclavian approach in such a way as 

 to produce a valvular effect. By this means the sac has acquired 

 its ultimate connection with the venous system (fig. 11, A, B, 

 and C). 



The systemic lymphatic drainage line. The transition from 

 the conditions of the systemic lymphatic organization of the 20 

 mm. embryo to those of the 23 mm. is characterized by a con- 

 fluence of the several previously established segments, with the 

 result that the thoracic duct is now a continuous channel and 

 at the same time has upon the left side, acquired connection with 

 the lymph sac. 



The azygos segment. This portion of the systemic lymphatic 

 line gives evidence of the least change of character. It is still 

 a capacious channel situated dorsal to the aorta, in places sur- 

 rounding the vessel. If changed at all, it has somewhat lost 

 in capacity, especially because its periaortic plexuses are less 

 rich and numerous. In relation to the azygos veins, it still is 

 interposed between them and the aorta. The plexuses which, 

 in the early conditions, were observed at its cephalic and caudal 

 extremities have given place to definite channels (fig. 12). From 

 the cephalic plexus there has arisen a single large trunk which 

 is paralleled by a second small lymph vessel, both of which pass 

 over into the pre-azygos segment. The caudal plexus develops 

 two channels which communicate with the post-azygos segment. 



Pre-azygos segment (fig. 12). Here the greatest change has 

 occurred, for the plexiform and irregular channels of the earlier 

 pre-azygos segment have become defined as two parallel trunks. 

 The larger of these is the more constant and apparently repre- 

 sents the main line of drainage. Upon the left side the confluence 

 of the several divisions of the pre-azygos segment has been 

 carried so far as to form a complete connecting vessel in com- 

 munication with the dorsal descending process of the lymph 

 sac of that side. On the right side this confluence is not as com- 



