14 DEVELOPMENT OF THE VEINS IN THE EMBRYO !'!<;. 



vein is indicated in the legend, but, it is only in a little older stage that it extends so far 

 caudalward (fig. 3). By the time the embryo is 11 mm. long the ventral vein extends the 

 full length of the organ and joins the mesial vein. It must, however, be pointed out that 

 it is only for a short time that the conditions of three complete longitudinal veins (extending 

 throughout the organ as shown in figure 3) remain. By the time the embryo is 12 or 13 

 mm. long the marked development of the mesial cardinal system as a part of the vena cava 

 has caused a breaking of the longitudinal veins, which remain as the main veins only for 

 the anterior third of the organ. 



THE POSTERIOR CARDINAL VEIN. 



Each of these three veins must now be considered in detail. The dorsal vein, which 

 is the original posterior cardinal vein, lies at first along the dorso-median border of the 

 Wolffian body and receives all the spinal veins. In view of its later history, however, it, 

 is noteworthy that there are variations from this primary position in embryos as early as 

 the 8 mm. stage. For example, in figure 2 the dorsal vein is deflected to the lateral surface 

 for a short distance in the lower half of the organ and then returns to the dorso-medial 

 border. In fact, though the dorsal vein is complete in this specimen, its caudal part was 

 injected through the medial vein. Opposite the area in which the dorsal vein does not lie 

 in the dorso-median angle the spinal veins drain into certain transverse veins. Another 

 more common variation than that shown in figure 2 is that the main dorsal trunk is deflected 

 to the lateral surface for the lower quarter of the organ. These variations explain why it is 

 not always possible to find the dorsal vein exactly in the dorso-median angle of the Wolffian 

 body throughout, even in stages as early as those measuring 8 to 10 mm. In fact, there is 

 a marked tendency for the dorsal vein to be deflected from the dorso-median angle, so that 

 in some specimens measuring 10 mm. the dorsal vein is so much deflected that it receives 

 only the most anterior of the spinal veins of the area corresponding to the Wolffian body. 

 This deflection of the posterior cardinal vein around to the lateral surface of the cephalic 

 part of the organ is found in practically all specimens of embryo pigs about 12 to 13 mm. 

 long. Its final position obliquely across the dorso-lateral surface of the organ is shown in 

 figure 4, plate 1, from an embryo 23 mm. long. The process by which the posterior cardinal 

 vein becomes limited to the cephalic end of the Wolffian body, as seen in figures 4 and .">. 

 is involved in a study of the mesial cardinal vein. 



THE MESIAL CARDINAL VEIN. 



The mesial cardinal vein is by far the most important of the veins of the Wolffian body. 

 By the time the embryo is 18 mm. long, it is already the largest vein of the organ. Refer- 

 ring to figure 2, the fluid injected into the anterior cardinal vein ran in three directions into 

 the liver, into the posterior, and into the ventral cardinal vein, while the most extensive 

 injection of the Wolffian body was through the medial cardinal vein by means of its anasto- 

 mosis with the posterior cardinal vein. The mesial cardinal vein lies obliquely across the 

 mesial surface of the Wolffian body, joining the posterior cardinal about one-third the 

 length of the Wolffian body from its anterior pole. At this stage the most important point 

 in the development of the mesial cardinal vein has been established, namely, that it has 

 formed the connection through the right caval mesentery with the posterior lobe of the 

 liver. The position of this connection is shown in dotted lines in figure 2, the lines being 

 drawn from an injection of another specimen from the same litter of pigs. The develop- 

 ment of this connection between the right mesial cardinal vein and the liver is well known 

 through the work of Minot, F. T. Lewis, and Davis. The present work serves to emphasize 

 more strongly the position of Minot, that the vena cava develops primarily to drain the 



