DEVELOPMENT OF THE THORACIC DUCT 135 



of a link of the chain, by gradual reduction of the central atro- 

 phying vein, to a point (sections 3 and 4, figs. 227 and 228,) where 

 the same stops, and only the perivenous lymphatic anlage (<5) con- 

 tinues. The latter also ceases blindly in the following section, 

 (5, fig. 229), and then follows an interval in which no lymphatic 

 anlages exist. 



These gradually appear again, in the same situation, and reach 

 their full development about the middle of this slide in sections 

 15, 16, 17, 19, 20 and 22 (figs. 214 to 219 inclusive). 



In the caudal part of the same slide both the central decadent 

 vein core (4) and the enveloping lymphatic space (5) again dimin- 

 ish, and, at the same time, approach the ventral aspect of the inter- 

 azygos plexus, in sections 26, 27, 28, 32, and 34 (figs. 220 to 224 

 inclusive). They continue in this condition in the beginning of 

 slide xv. Further caudad the lymphatic anlage again increases 

 in size and rapidly reaches its former degree of development. 

 Thus section 10 of slide xv (fig. 230) corresponds to section 15 of 

 slide xiv (fig. 214) in the extent and relations of both the central atro- 

 phying venous core (4) and the enveloping extraintimal lymphatic 

 space (5], and the same condition is maintained in the succeed- 

 ing sections 11, 14, 16 and 17 of slide xv, (figs. 231, 232, 233 and 

 234) which compare strictly with sections 16, 17, 19, 20 and 22 of 

 slide xiv, (figs. 215, 216, 217, 218 and 219). 



Sections 10 and 11 of slide xv (figs. 230 and 231) show especially 

 well the relation of the atrophying central venule (4) and the sur- 

 rounding extraintimal lymphatic space (5} to the right azygos 

 trunk ($), the aorta (7) and the right intersegmental artery (2). 

 Comparison, for example, with corresponding sections of the pre- 

 ceding series 76 will demonstrate the identity of both anlages in the 

 two embryos (cf. series 76, slide xi, sections 16, 17 and 19, (figs. 

 207, 208 and 209). 



The relations of the early thoracic duct anlages are clearly evi- 

 dent in the ventral view of the reconstruction of embryo 214, 

 shown in fig. 190. 



The lymphatic anlage of the left side, forming the beginning 

 of the caudal part of the left thoracic duct, smaller and less 

 developed in this stage, compared with the corresponding struc- 



