RELATION OF LYMPHATIC TO BLOOD- VASCULAR SYSTEM 31 



phatic plexus, as development proceeds, from the 12 mm. stage 

 on, approaches the large venous trunks more and more, until 

 the spaces lie in direct apposition with the same, and unite to 

 form the lymphatic trunk eventually destined to replace the left 

 azygos arch and adjacent position of the left precardinal vein. 

 This trunk then constitutes the cephalic end of the broncho- 

 mediastinal duct (37). Thus figs. 13 and 14 show two sections 

 through the same region in a 15 mm. embryo (series 219, slide 

 xiv, sections 19 and 16). Compared with the 14 mm. embryo 

 .the left azygos vein shows a marked reduction. The lym- 

 phatic spaces have enlarged and present a clear lumen on 

 section, the remnant of the earlier vein, around which they 

 developed, having disappeared. The spaces lie between aorta 

 and left precaval vein, in close approximation to the dorso-medial 

 circumference of the latter. The azygos segment of the thoracic 

 duct (86) is seen dorsal to the interval between aorta and oesoph- 

 agus, and ventral to the scant remnants of the earlier interazygos 

 venous anastomosis (15). 



The reconstruction of a 15 mm. embryo (series 218) shown in 

 fig. 170 in Part II, gives a clear idea of the extent and relations of 

 this lymphatic complex (37) in this stage. 



B. Partial replacement of portions of the territory of an early 

 embryonic venous pathway by an extraintimal lymphatic vessel, both 

 venous and lymphatic channels either persisting side by side up to 

 later developmental periods, or forming correlated components of 

 the permanent adult vascular organization. 



The developmental processes just described appear most clearly 

 marked in the earlier stages, and in connection with temporary 

 embryonic channels and plexuses which are destined to undergo 

 rapid degeneration and ultimate complete elimination. In the 

 case of the embryonic veins which are retained for a longer period, 

 or carried over into the permanent adult organization, the his- 

 togenetic stages of lymphatic development are identical in kind, 

 and differ only in degree from those just detailed. In place of 

 complete replacement of the antecedent vein by the lymphatic 

 channel, this replacement is only partial and leads to the typical 



