270 ANNALS NEW YORK ACADEMY OF SCIENCES 



the azygos as a new or secondary vein retaining only the termination of 

 the postcardinal as its point of debouchment. Unfortunately in the fol- 

 lowing year Eathke modified his view and considered that the azygos was 

 of postcardinal origin to the 8th or 10th intercostal space and only beyond 

 that was what we should now term supracardinal. This error taken up 

 by Huntington has obtained wide currency, though it has been corrected 

 by Zumstein (1897) in the guinea-pig, by McClure in the opossum 

 (1902) and by Sabin (1915) in the pig. 



Among adult mammals the azygos may be bilateral as in monotremes 

 and many diprotodont marsupials. It may be absent as in cetacea (v. 

 Baer), in Cholcepus and Bradypus (Hochstetter). It is usually right- 

 sided, but may be left, with or without the persistence of a left cava. 

 Marshall and more recently Beddard have listed its conditions in many 

 species. It is right-sided in Primates, Carnivora, Perissodactyla, Xenar- 

 thra except the sloths, in Hyrax and in Tragulus, It is left-sided in 

 many polyprotodont marsupials, in Suidse, in Moschinse and in hollow- 

 horned ruminants. Among rodents the right azygos is the rule; it is 

 associated with a smaller left azygos in Hystrix, while in the beaver the 

 vein of the left side alone persists. A specimen of this animal re- 

 cently dissected at Columbia confirms Beddard's interesting observation. 

 In general, bilaterality of the azygos is associated with low position 

 (Beddard). 



Dollinger, v. Baer and the earlier students of the vascular system gen- 

 erally were familiar with the plexiform character of the embryonic blood 

 vessels and appreciated the role of hydrodynamic factors in formation of 

 trunks by the selection of certain lines of flow from the multiplicity of 

 possible channels afforded by the plexuses. Since Eoux and Thoma the 

 appreciation of the mechanical factor has become general and the impor- 

 tance of the drainage area in determining the proximal channel has been 

 recognized. The peripheral plexus increases with the size of the part 

 in which it lies, and with the differentiation of the part tends to acquire 

 independence from the general plexus. The venous trunks become the 

 indexes of the development and growth of their drainage areas and the 

 pattern of the great veins is, as it were, the expression and integration 

 of the specialization and relative growth rates of the various parts of 

 the body, and is relative to the embryonic condition, a reduction and sim- 

 plification, serving to promote mechanical efficiency by the diminution 

 of surface friction by the substitution of a few large trunks for the 

 numerous plexiform channels. As this is accomplished the general 

 venous plexus becomes resolved into independent districts connected ulti- 



