28 DEVELOPMENT OF THE VEINS IN THE EMBRYO PIG. 



becomes incorporated in the Wolffian body and disappears with that organ. Subsequently 

 the lower segment of the vena cava becomes unilateral. The fact that the lower segment 

 of the vena cava comes from a prevertebral plexus has also been observed by Huntington 

 and McClure (1907) in a study of the development of the veins in the cat. They have 

 called the prevertebral plexus a supracardinal system of veins. 



The entire point at issue in the development of the azygos system is the place of its 

 permanent connection with the cardinal system. The belief has been that it lies opposite 

 the tenth vertebra, but as a matter of fact it lies at the duct of Cuvier. The crucial point 

 for distinguishing the azygos veins from the posterior cardinal veins lies in the more dorsal 

 position of the azygos veins. It has been shown that the azygos plexus has a great number 

 of connections with the cardinal system, anterior as well as posterior. At first the cardinal 

 veins are enormous and the azygos veins tiny. At last there comes a stage in which there 

 are two veins of equal size draining into the duct of Cuvier, a dorsal vein or the azygos, 

 and a ventral vein or the posterior cardinal. This is clearly shown in figure 15, plate 6, 

 from an embryo measuring 22 mm. The fact that the azygos vein is farther dorsal than 

 any. portion of the posterior cardinal vein is the crucial point in distinguishing the two 

 veins in sections. This is best shown in figure 5. Since the posterior cardinal vein becomes 

 incorporated in the Wolffian body it necessarily lies as far ventral as that organ. Starting 

 from the stage of from 21 to 22 mm., when the two veins are of the same size, the more 

 dorsal vein (the azygos) increases in size, especially at the anterior end, and the more ven- 

 tral vein begins to be dragged caudalward, becoming a tributary of the azygos vein. This 

 is well shown in figure 4, from an embryo 23 mm. long. On the right side the dorsal vein 

 (or the azygos) is the only one of the two which connects directly with the duct of Cuvier. 

 Without a figure of the preceding stage in which the two veins are equal (fig. 15, plate (5) 

 it would not be easy to prove from figure 4 alone that the posterior cardinal is really now a 

 tributary of the azygos vein. The point, however, is readily proved from serial sections, 

 because in them it is perfectly clear that it is the more dorsal of the two veins that finally 

 joins the duct of Cuvier. In the specimen from which figure 4 is taken the azygos vein is a 

 distinct vessel up to a point half-way between the two leaders v. A', and v. c. p., where the 

 more dorsally placed azygos vein receives the posterior cardinal vein. The deep shadow on 

 the vein obscures this point in the figure. 



The right posterior cardinal vein then joins the azygos in figure 4 at the point where 

 the azygos curves outward to join the duct of Cuvier. Comparing this with figure 15, 

 where the two veins are equal and both enter the duct of Cuvier, it is plain that in the stage 

 of figure 4 the posterior cardinal is being dragged backward, becoming thereby a branch 

 of the azygos. On the left side of figure 4 the hemiazygos and posterior cardinal both join 

 the duct of Cuvier, showing that the left side is not quite as far developed as the right. 



The more dorsal position of the hemiazygos is especially clear in figure 4. The funda- 

 mental points of the azygos system are all shown in this figure. Both the right and the left 

 azygos veins occupy a dorsal position and empty into the duct of Cuvier. The ladder- 

 like pattern of the two azygos veins, which was noted by Hochstetter, is very clear. Drain- 

 ing into the azygos vein is a small descending vein which I shall call the right oblique vein. 

 Its meaning and further development are clear in figures 17 and 18, plate 7. It drains the 

 prevertebral tissues of the first four segments in the pig and is about as long as the accessory 

 hemiazygos (fig. 17). It is usually larger than the hemiazygos and thus drains more 

 than half of the prevertebral tissue. In figure 16 it is, however, slightly smaller than the 

 hciniazygos. It is not the superior intercostal, which lies farther forward and drains into 

 the azygos at the duct of Cuvier. It is a special branch of the azygos vein characteristic 



