DEVELOPMENT OF THE VEINS IN THE EMBRYO I'Ki. 1 ii 



As has just been stated, there are at first hut two veins in the Wolrllan body, a posterior 

 and a mesial cardinal, and these form a common stem, the direct, continuation of t In- 

 posterior cardinal which connects with the duct of ( 'uvier. This condition is found in 

 embryo pigs measuring 6 to 7 mm., when the mid-brain curve is the only curve of the 

 cephalic end of the embryo, and the tail is not curved at all. The next stage in the develop- 

 ment of the veins, as seen in figure 1, is characterized by three longitudinal veins, showing 

 still the most important primitive relation of the veins, namely, that the mesial cardinal 

 vein drains into the posterior cardinal. Figure 1 is from an injection with silver nitrate 

 made into the anterior cardinal vein of a pig embryo 9 mm. long. It is clear that, even at 

 this early stage, the mesial cardinal vein, though not as long as the posterior cardinal vein, 

 is distinctly wider. The posterior cardinal vein runs the full length of the Wolffian body 

 and is continued into the caudal vein. The Wolffian body is represented as transparent 

 and the mesial vein is seen through it, lying along the ventro-mesial border of the organ. 

 The mesial cardinal vein is connected by many large superficial mesial vessels with the 

 posterior cardinal. These can be seen as distinct vessels only in the posterior part of 

 figure 1, for in the anterior part they are obscured by the extensive capillary injection within 

 the Wolffian body itself. The mesial cardinal vein curves dorsalward to join the posterior 

 cardinal vein at a point somewhat caudal to the cephalic pole of the Wolffian body. 



The third longitudinal vein, the ventral or ventro-lateral, begins at the anterior pole 

 of the organ, where it joins the posterior cardinal vein. At the stage of figure 1 it is the 

 smallest of the three long veins. In its course it follows the path of the Wolffian duct lying 

 just mesial to it. It is represented in the figure as if cut off, but in the specimen it breaks 

 up into capillaries a short distance below the point at which it is shown cut off. It is clear 

 that this ventro-lateral vein is connected by transverse veins with both the posterior and 

 the mesial cardinal veins. 



As the tail of the embryo begins to curve and the limb buds develop, certain very 

 important changes take place in the veins of the Wolffian body. These are shown in figure 

 2, plate 1, from an injection of silver nitrate into the veins of an embryo measuring 8 

 mm. in its longest diameter, and in figure 3, from a similar injection in an embryo meas- 

 uring 1 1 mm. The further development of the ventral vein, to the caudal pole of the organ, 

 where it also connects with the mesial cardinal veins, establishes the three longitudinal 

 veins of the Wolffian body. In general these three main veins lie just beneath the capsule, 

 and the posterior cardinal vein has as its primary position the dorso-median angle of the 

 Wolffian body along the edge of the body-cavity, while the mesial vein lies medial to the 

 nephritic arteries and the lateral vein lies just mesial to the Wolffian duct. 



The position of these three veins can be recognized in section in figure 6, plate 3, which 

 is from an embryo pig 9.5 mm. long, about the stage of figure 2. In general, such a survey 

 of the vessels of an organ as is given in figure 2 is exceedingly valuable in studying the 

 vascular system by means of serial sections in fact, by an easy method and without the 

 chances of error, it gives the results of reconstruction. For example, if imaginary sections 

 are taken through figure 2, it is clear that a longitudinal vein in one section must appear as 

 a part of a transverse vein at the next level. 



The section of figure 6 is through the lower half of the Wolffian body just below the 

 level of the ventral vein as seen in figure 2. It shows the posterior cardinal vein in its 

 primitive position in the dorso-median angle of the Wolffian body and at the edge of the body- 

 cavity. The mesial vein is in the characteristic position mesial to a nephritic artery. The 

 section shows two large transverse veins, one connecting the mesial and posterior cardinal 

 veins and the other along the ventral surface of the organ. The position of the ventral 



