SUMMARY AND CONCLUSIONS 169 



The 14 mm. embryo represents thus in general the full develop- 

 ment of the histological picture of secondary replacement of a 

 decadent embryonic venous plexus by the secondarily developed 

 extraintimal perivenous lymphatic spaces and channels. 



The following conditions have been noted in individual sec- 

 tions : 



a. Ventro-medial tributary of right azygos connected with main 

 vein, detached on left side. 



Anlage of right thoracic duct lateral to right ventral azygos 

 tributary . 



b. Ventro-medial tributaries of both azygos veins surrounded by 

 extraintimal anlages of both ducts. 



c. Ventro-medial tributaries detached and surrounded by ex- 

 traintimal anlages of thoracic ducts on both sides. 



d. Ventro-medial tributaries in connection with both azygos 

 veins and accompanied by ventro-lateral lymphatic anlages on 

 both sides. 



e. Azygos veins without ventro-medial branches or their rem- 

 nants and accompanied on both sides by the thoracic duct anlages 



/. Azygos veins with ventro-medial tributaries without duct 

 anlages. 



g. Azygos veins without either ventro-medial tributaries or 

 thoracic duct anlages. 



D. Later transitional lymphatic stages. Illustrations in body of 

 text: figs. 252 to 258. 



In the 15 and 15.5 mm. embryo three further developmental 

 characters appear: 



a. The earlier more redundant and plexiform lymphatic anlages 

 condense into a definite channel of relatively smaller caliber. 



b. The central remnants of the atrophied embryonic vein, 

 around which the lymphatic anlages developed, have to a large 

 extent disappeared, leaving the lymphatic lumen clear. 



c. The individual and originally separate segments of the lym- 

 phatic channels have united into segments of greater length, and 

 have approached more closely to the ventral surface of the main 

 azygos trunks. The definition of a right and left lymphatic trunk 

 with supra-aortic cross anastomoses is beginning to appear, al- 



