DEVELOPMENT OF THE VEINS IN THE EMHKYO !'!<;. 1") 



Wolffian both'. The mesial cardinal vein becomes the chief vein of the Wolff inn body and 

 thus the vena cava forms to drain that organ. It is true that, even as early as the stage 

 represented in figure 1, the mesial cardinal vein has tiny tributaries in the mesentery and 

 subsequently the territory of direct drainage into the vena cava is increased through taking 

 over the common trunk of the caudal vein and the primitive fibular vein, but primarily it 

 drains practically only the Wolffian body. 



The two mesial cardinal veins anastomose freely across the mid-line ventral to the 

 aorta. At the stage of figure 2, in embryos 8 mm. long, there are two large anastomoses, 

 the anterior one being just caudal to the junction of the mesial and posterior cardinal veins. 

 Subsequently there may be five or six anastomoses, which finally fuse to make the median 

 mesonephritic vein of Minot. This abundant anastomosis accompanies the unilateral 

 development of the vena cava in the right caval mesentery. 



THE ANTERIOR MESIAL VEIN OF THE WOLFFIAN BODY. 



Just at the anterior pole of the Wolffian body there is a short mesial vein lying between 

 the dorsal and the ventral veins (fig. 2). This short vein does not connect with the main 

 mesial or subcardinal vein, but rather with the posterior cardinal vein. It drains the 

 mesial surface of the anterior pole of the organ and is present and constant on both sides. 

 The vein is important because it is constant and remains for a considerable time (fig. 4, 

 plate 1, and fig. 10, plate 4) and hence must be distinguished from the developing azygos 

 vein, which will be brought out later. Its fundamental difference from the mesial cardinal 

 vein is that it retains its connection with the posterior cardinal system and never joins 

 the vena cava, whereas a comparison of figures 2 and 3 shows that the circulation through 

 the mesial cardinal vein is gradually thrown entirely through the liver. 



THE VENTRAL VEIN OF THE WOLFFIAN BODY. 



The third longitudinal vein of the Wolffian body, the ventral vein, eventually extends 

 the full length of the organ (fig. 3) and joins the two other veins at the caudal pole, while it 

 joins the posterior cardinal vein at the anterior pole of the organ. In fact, the posterior 

 cardinal vein might be said to bifurcate at the anterior pole of the organ into a dorsal and 

 a ventral branch. This is well known and was figured by Minot. The ventral vein extends 

 along a ridge on the ventral surface of the Wolffian body, in which lies the Wolffian duct- 

 In a general way this ridge marks a boundary between a mesial glomerular zone and a lat- 

 eral tubular zone, as seen from the ventral aspect (fig. 4); the vein lies just mesial to the 

 Wolffian duct (fig. 6). In early stages the ridge extends along the ventral border of the 

 organ, but as the tubules become convoluted and so take up more space it becomes a curved 

 line on the ventral surface (fig. 4). 



THE TRANSVERSE VEINS OF THE WOLFFIAN BODY AND THE VENA CAVA. 



Having described the longitudinal veins of the Wolffian body, it is now necessary to 

 take up the transverse superficial veins, which are both characteristic and important . These 

 veins have not yet been adequately emphasized, so far as I am aware. The longitudinal 

 veins are connected by two sets of transverse veins, the lateral and the mesial. Both are 

 present from the very beginning of the formation of the posterior and mesial cardinal veins 

 from the lateral branches of the aorta and have from the first a characteristic pattern. The 

 lateral transverse veins connecting the dorsal and ventral veins are small, regular, anil in 

 the beginning alternate with the straight transverse tubules of the organ, so that there are 

 as many transverse veins as there are tubules. An injection of the tubules with silver 

 nitrate gives the negative pattern to that of the transverse veins shown in figure 3. These 



