32 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



close parallel association of lymphatic and venous vessels so 

 characteristic of the later embryonic stages and of the per- 

 manent adult pathways of both the lymphatic and the venous 

 systems. 



Figs. 15 and 16 show transverse sections of a 14 mm. embryo 

 (series 222, slide vii, section 26, and slide viii, section 4) in the 

 region of the external jugular vein. The lymphatic spaces (27') 

 are in full development, and are applied chiefly to the medial 

 aspect of the plexus of the external jugular vein (27). The lym- 

 phatic endothelium is clearly marked. 



Fig. 17 shows a transverse section in the upper thoracic region 

 of another 14 mm. embryo (series 37, slide xiii, section 12). A 

 typical lymphatic anlage (53) is applied to the medial wall of the 

 left precaval vein. 



Fig. 18 shows a transverse section through the mid-thoracic 

 region in a 17 mm. cat embryo (series 258, slide xviii, section 

 9, X 225). 



The extraintimal anlagesof the thoracic ducts (86), which usually 

 in thisstage have advanced to the production of a continuous and 

 uninterrupted channel system, are seen on each side closely ap- 

 plied to the ventral aspect of the left and right azygos veins. 

 The latter are in the height of their development, forming large 

 and symmetrical longitudinal venous trunks (3, 6) connected by 

 the supra-aortic interazygos anastomosis. In course of further 

 development the left azygos vein and the interazygos anastomosis 

 are destined to undergo progressive reduction until they are 

 eventually entirely lost. Their topographical position is then 

 occupied by the replacing left segment of the thoracic duct com- 

 plex (36). The right azygos vein, which is carried as a perma- 

 nent vessel into the adult organization, also undergoes consider- 

 able relative reduction, correlated to the corresponding increase 

 in the caliber of the main (right) segment of this portion of the 

 thoracic duct complex. 



The beginning of this process is seen well in fig. 19, which shows 

 a transection of the same region in a 19 mm. cat embryo (series 

 253, slide xxiv, section 9, X 225). The change from the preceding 



