80 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



lar, condenses and becomes more clearly defined against the sur- 

 rounding mesoderm. The multilocular character due to rem- 

 nants of the capillary walls in the interior, disappears and the 

 approach obtains a clear and clean-cut lumen. The entire struc- 

 ture appears to contract somewhat on itself and develops more 

 distinct walls. The recession, during which the thoracic duct 

 approach occupies a relatively smaller territory is succeeded by 

 3. Final stage. Embryos of 15-17 mm. 



Series 245 15 mm. 



Series 218 15 mm. 



Series 143 15.5 mm. 



Series 142 17 mm. 



In this period the sac, through its digitate processes, establishes 

 the definite connections with the independently formed systemic 

 extraintimal lymphatic channels. Specifically the thoracic d uct 

 approach of the sac unites secondarily with the preazygos seg- 

 ment of the thoracic duct and with the broncho-mediastinal 

 lymphatic trunk. 



While I do not insist on the above outlined details in the 

 chronological order of development of the thoracic duct approach, 

 the embryos of my collection certainly tend to support the view 

 expressed. Thus the 13 mm. embryo (series 92) offers the 

 highest degree of expansion of the thoracic duct approach. The 

 structure recedes and diminishes somewhat in extent in the 

 13.5 mm. stage (series 189), appears uniformly further reduced 

 and shortened in the 14 mm. embryos (series 210, 212, 122), and 

 finally, from the 15 and 15.5 mm. stages on, makes its definite 

 connections with the preformed independently developed chan- 

 nels of the thoracic and broncho-mediastinal ducts. 



In this entire genetic process there is not the slightest indica- 

 tion of an extension of lymphatic trunks from the jugular sac or 

 from its thoracic duct approach toward the periphery. There 

 is absolutely no suggestion of so-called " budding" or " sprout- 

 ing," or of any other form of centrifugal extension of lymphatic 

 vessels. On the contrary, the sac as a whole, and the processes 

 therewith connected, recede actually for a time, and only then 

 establish a secondary junction with the independently developed 



