3 io 



HISTOLOGY 



such a loop is found in all classes of vertebrates. Venous blood entering 

 the Wolffian body posteriorly flows out from it anteriorly, and circulates 

 among the tubules in lacunar vessels, closely resembling the hepatic sinu- 

 soids. This is the "renal portal system." It should be noted, however, 

 that the renal sinusoidal vessels are poorly developed in mammalian 

 embryos. 



In sections these veins are readily recognized. The mesonephric 

 arteries pass from the aorta to the glomeruli of the Wolffian body, between 

 the subcardinal vein in front and posterior cardinal vein behind (Fig. 

 303, C). In places, the subcardinal veins form large anastomoses across 

 the mid-ventral line; the posterior cardinal veins are further apart, and 

 receive intersegmental branches from the dorsal musculature. 



As the kidneys grow upward behind the Wolffian bodies, their ureters 

 become encircled by a branch from the posterior cardinal vein (Fig. 305, 

 A). The venous loop around the ureter was described by Hochstetter 

 (Morph. Jahrb., 1893, vol. 20, pp. 543-648), and its dorsal limb, together 

 with secondary anastomoses, has been named the supracardinal vein 

 (Huntington and McClure, Anat. Rec., 1907, vol. i, pp. 29-30). The 

 transformation of these veins into the branches of the inferior vena cava 

 is represented somewhat diagrammatically in Fig. 305, B, and may be 

 briefly described as follows: 



The anastomosis between the subcardinal veins becomes a part of the left renal 

 vein. Above this anastomosis the right subcardinal vein connects with the veins of the 

 liver and forms a portion of the vena cava inferior. The left subcardinal vein, above 

 the renal anastomosis, becomes reduced to the left suprarenal vein (Hochstetter). The 

 subcardinal veins below the renal anastomosis are associated with lymphatic vessels to 

 which they apparently give rise; otherwise they disappear. 



The posterior cardinal veins above the renal anastomosis, after they have been 

 tapped by the formation of the vena cava inferior, are known as the azygos and hemi- 

 azgos veins, and the outlet of the left common cardinal becomes cut off as the coronary 

 sinus (Fig. 305, B, which shows also the formation of the superior vena cava). Below 

 the renal anastomosis the posterior cardinal veins give rise to the genital veins (sper- 

 matic or ovarian), and the Wolffian body becomes reduced to an appendage of the geni- 

 tal glands. As the genital glands descend into the pelvis, their veins become elongated; 

 and the corresponding arteries, derived from the mesonephric arteries, are likewise 

 elongated. The supracardinal vein on the right side becomes a part of the vena cava 

 inferior; on the left it is probably represented by the ascending lumbar vein. 



The kidneys are supplied by vessels which enter them after they have attained their 

 permanent position. Their arteries and veins consequently pursue a straight course 

 from the aorta and vena cava, respectively, to the hilus of the kidney. 



KIDNEY. 



Development. The kidney develops after the Wolffian body has been 

 formed. It arises in two parts, one of which is an outgrowth of the Wolf- 

 fian duct; the other is a mass of dense mesenchyma surrounding this 

 outgrowth, and said to be derived from the posterior nephrotomes. Both 



