SUMMARY AND CONCLUSIONS 



161 



sal aspect of the main vein and between the thyro-cervical artery 

 and sympathetic nerve, which has been alone employed as the 

 basis of the conclusion that, in comparing the 14 mm. and 16 mm. 

 embryo, the thoracic duct develops from a portion of the dorso- 

 medial plexus, secondarily split off from the main vein. 



Thus, for example, the series 122 under discussion, if employed 

 for purposes of topographical comparison with a 16 mm. embryo, 

 should be represented by the following sections: 



1. It will be seen that in section 21 of slide ix (fig. 57), the inter- 

 val between thyro-cervical artery (24) and sympathetic nerve (1) 

 is occupied by the thoracic duct approach (12} directed meso-cau- 

 dad from the jugular lymph sac. 



2. In section 28 of the same slide (fig. 58), the thoracic duct 

 approach (12} has separated from the sac, and appears, as the free 

 terminal of the future adult duct, mesad to the thyro-cervical 

 artery (24) > Further dorso-mesad is a venous tributary (16} 

 approaching the main vein. Elements of the mesal peri- and niter- 

 neural venous plexus are also seen. 



3. Section 7 of slide x (fig. 59) shows, in the interval between the 

 thyro-cervical artery (2Jf) and the sympathetic nerve (1} the sec- 

 tion of the thoracic duct approach (12} close to the dorsal circum- 

 ference of the common jugular vein (26}. 



4. In section 9 of the same slide (x) (fig. 60), the blind end of 

 the thoracic duct approach (12} is still seen on the mesal side of 

 the thyro-cervical artery (24), while further mesad the common 

 jugular vein (26} receives the terminals of the dorso-medial venous 

 plexus. Here, in other words, the Venous outgrowths' or 'an- 

 lages of the thoracic duct/ and the thoracic duct approach of the 



