23 G The Development of the Vena Cava Inferior 



A notable change is in the decreasing importance of the inferior part 

 of the snbcardinal veins. Only two of its cross connections remain, 

 of which one, X, has become very large. A cross section, Fig. 8, of 

 this embryo, taken a short distance above this region shows the sym- 

 metrical arrangement of the vessels, and the large size of the subcardi- 

 nals on both sides. The snbcardinals lie at the ventral corner of the 

 hilns of the "Wolffian body from which they receive tributaries, just as 

 do the cardinals at the dorsal corner. The veins are separated from one 

 another by the mesonephric arteries, a pair of which is seen in the 

 section. At the upper end of the veins, on either side, cardinal and 

 subcardinal anastomose in condensed mesenchyma probably connected 

 with the suprarenal anlage. 



In the rabbit of 8.8 mm., the kidney on either side was situated in 

 front of the iliac artery, as described by Hochstetter, and beautifully 

 draAvn in his Fig. 18 of PI. XXII (93). As it develops, it drops back 

 over the artery and falls between the cardinal vein and the aorta, or even 

 directly upon the cardinal. It may split the cardinal vein so as to form 

 a loop, as figured by Hochstetter, but I have not seen any complete loop. 

 In Fig. 6, PI. II, the position of the kidney is indicated by E. A 

 portion of the cardinal vein receiving two intersegmental veins has been 

 separated from the main trunk and pushed dorsad. On the right side 

 of the same embryo. Fig. 5, PI. II, the vein was not divided, but the 

 kidney had distorted the course of two intersegmental veins. The main 

 cardinal stem bends rather sharply outward around the obstructing 

 kidney and so comes to lie on the outer side of the lower end of the 

 Wolffian body.^ The ureter is now on the median side of this large trunk. 

 From the shattered inner pieces of the cardinal vein, or from new off- 

 shoots of the main stem, a venous connection forms on the median side 

 of the ureter. Such a loop is seen in Fig. 5, the letter U marking the 

 passage for the ureter. This new median arm of the loop is in line 

 with the main vessel; it enlarges and becomes a part of the cardinal 

 trunk. The vein has again become straight, but the ureter has been 

 transferred from its inner to its outer side. The outer arm of the 

 loop becomes smaller, and its caudad portion is divided into many sinu- 

 soids. It then appears as a large branch of the cardinal vein, entering 

 it from the dorsal border of the Wolffian body. Thus it forms the ITr- 

 nierenvene of Hochstetter (93, p. 583), The ureter remains in the 

 loop and passes, therefore, ventral to the iliac artery, external to the 



^In pig embryos of 12.0 mm. the main cardinal vessel passes to the outer side of 

 the caudal end of the Wolffian body uninfluenced by the renal anlage. 



