2() DEVELOPMENT OF THE VEINS IN THE EMBKYO PIG. 



Hoehstetter that they are longitudinal anastomoses of the spinal veins, but it is incor- 

 rect in regard to the relations of the new azygos veins to the posterior cardinals, for the 

 permanent connections are not opposite the tenth rib, but at the duct of Cuvier on both 

 sides. If both ducts of Cuvier are permanent, both right and left azygos veins open into 

 the heart through them in the adult, as is the case in the pig. If, as in the human, the left 

 duct of Cuvier disappears, the left azygos vein drains into the heart by means of anastomoses 

 with the corresponding vein of the other side. 



Since the work of Zumstein and of Parker and Tozier emphasizing the new preverte- 

 bral vessels as forerunners of the azygos veins, there have been two different views in regard 

 to the relation of these new veins to the posterior cardinal veins. McClure stated in 1906 

 that in the marsupials the azygos vein was formed from the new vein plus the proximal end 

 of the posterior cardinal vein. In 1912 Kampmeier expressed the view that, in the cat, the 

 portion of the azygos vein between the duct of Cuvier and the \Yolffian body was made by a 

 longitudinal fusing of the new vein with the posterior cardinal vein. The present study 

 demonstrates that in the pig the posterior vein disappears entirely and does not enter into 

 the formation of the azygos vein. 



CIRCULATION OF A TYPICAL BODY-WALL SEGMENT WITH RELATION TO THE AZYGOS VEINS. 

 In order to follow the origin of the azygos veins it is necessary to consider the general 

 vascular pattern of the body-wall in the stages that precede them. The azygos veins begin 

 in the embryo pig in stages measuring from 13 to 14 mm. If we consider the development 

 of a typical body-segment up to this stage, it will be remembered that Evans, in his chapter 

 in the Manual of Human Embryology, Keibel-Mall, 1912, brings out the fact that the 

 primitive circulation of a body-wall segment consists of a medial spinal artery, a capillary 

 plexus in the pia, and a lateral spinal vein. In the development of the vascular system of 

 the body -wall in the mammal there are two sets of arteries, a dorsal and a lateral. The 

 dorsal arteries, which are intersegmental, pass to the cord, and establish a longitudinal 

 anastomosis along its lateral surface. The lateral arteries arc not segmental, but also 

 establish a longitudinal anastomosis in the Wolffian groove. This longitudinal vessel 

 becomes connected with the posterior end of the heart and makes the primary venous 

 system. The lateral arteries, except those going to the glomeruli of the Wolffian bodies, 

 then disappear. When the dorsal segmental arteries connect by segmental branches with 

 the cardinal vein the embryonic venous arch of a typical body-wall segment is established 

 and this is the embryonic arch of Evans, dorsal segmental artery, capillary plexus of the 

 pia, spinal vein to the cardinal vein. This arch can not be said to be primary in the sense 

 of origin, since the dorsal segmental vessels do not give rise to the cardinal vein in the 

 mammal. In other words, the first veins in the body-wall of the mammalian embryo are 

 longitudinal chains from the lateral arteries and it is only when the dorsal or segmental 

 arteries connect with this vein that there is a segmental vascular arch. The capillary 

 circulation of the cord consists, as Evans has shown, of a primary capillary plexus along the 

 dorso-lateral surface of the cord, and of a secondary ventral plexus along the surface of 

 the cord in which the spinal arteries pass to the anterior spinal artery. The primitive 

 dorsal segmental arteries fork into a ventral and a lateral branch just between each 

 ganglion and the cord. Besides the two branches of the spinal artery, ventral and lateral, 

 each ganglion receives one or more tiny arteries (fig. <>). 



From the standpoint of the development of the azygos veins, the part of the vascular 

 arch of a body-segment that is of most interest is the spinal vein. Moreover, the important 

 point is the position of these veins. As can be seen in figure 6, the spinal veins lie wholly 



