88 F. W. THYNG 



intersegmental veins. Opposite the 1st lumbar spinal nerve 

 it bends ventrally, and is continued as a large dorso-ventral 

 channel (compare Sabin '09, fig. 11), which terminates in the 

 caudal extremity of the part of the subcardinal which still per- 

 sists. This dorso-ventral segment of the supra-ureteral channel, 

 extends medial to the metanephros and Wolffian body from 

 both of which it receives tributary veins. One (or more) of 

 these tributaries undoubtedly forms the right renal vein (or 

 veins) . The supra-ureteral channel forms a considerable portion 

 of the definitive inferior vena cava. 



The subcardinal veins are derived from Wolffian sinusoids 

 in the region ventral to the mesonephric arteries (Lewis '02). 



x4.t this stage the right subcardinal (V.scard.d.) begins at the 

 renal anastomosis (x), which is formed by the confluence of the 

 sub- and supra-ureteral channels from both sides of the body. 

 From the renal anastomosis the right subcardinal vein extends 

 cephalad, ventral to the right suprarenal gland (Gl.s.-r.d.), as a 

 component of the vena cava inferior. 



By the development of the suprarenal gland the cephalic part 

 of the V. subcardinalis (which previously opened into the posterior 

 cardinal vein on a level now represented by the cephafic extremity 

 of the gland) has been reduced to sinusoidal channels. The 

 cephalic end of the portion of the right subcardinal vein, now 

 persisting, represents the anlage of the right suprarenal vein 

 (V.s.-r.d.) which connects, as does the azygos, with the suprarenal 

 .sinusoids. 



It should be stated here that the presence of sinusoids in the 

 suprarenal glands of mammals and other vertebrates, has been 

 demonstrated histologically by Minot ('00). 



Also significant in this connection are the observations of 

 Lewis ('02, p. 236) upon the veins in question in a rabbit embryo, 

 11 mm. in length. He says "at the upper end of the veins, on 

 either side, cardinal and subcardinal anastomose in condensed 

 mesenchyma probably connected with the suprarenal anlage." 



The large renal anastomosis {x), referred to above, unites the 

 two subcardinals. It crosses the median line, ventral to the aorta, 

 a little caudad of the superior mesenteric artery (A.wes.s.) of 



