272 



venous system of the left Wolffian body of this embryo seen from the 

 ventral side. The caudal vein is continued as the mesonephric azygos, 

 connecting by many sinusoids with the subcardinal vein. Anteriorly 

 the azygos vein leaves the Wolffian body and proceeds to the heart. 

 Its course, already hard to trace among the sinusoids, is soon inter- 

 rupted. In the 14.5 mm embryo, Fig. 9B, the azygos vein no longer 

 leaves the Wolffian body anteriorly. The kidney has passed relatively 

 upward behind the Wolffian body, to its final position, and small veins 

 enter its pelvis. The Wolffian body, swinging out laterally, is rapidly 

 losing the connections between its sinusoids and the subcardinal and 

 azygos veins. (The posterior part of the subcardinal, having lost all 

 its connections with the veins figured, has been omitted from the 

 drawing.) The loop around the ureter has developed as described by 

 HocHSTETTER ^), SO that the main azygos channel has been transferred 

 to the inner side of the ureter. The part of the old channel within 

 the Wolffian body receives most of the sinusoids. Their regression 

 continues with that of the mesonephros. In the 29 mm rabbit. Fig. 9 C, 

 they have become small veins, tributaries of the genital vein, which 

 has come from the mesonephric azygos and its subdivisions. Some 

 sprouts from the sinusoids in this embryo enter the genital gland. 

 The mesonephric arteries have been reduced in number, and from one 

 of them, sometimes two or three, arterial sprouts also enter the genital 

 gland. There they connect with the veins by a capillary system. When 

 this is accomplished the mammalian renal portal system has ceased to 

 exist. Its sinusoids have become a part of the genital vein, and may 

 possibly account for the many subdivisions constituing the pampiniform 

 plexus. 



In the Torpedo and Necturus the genital veins are tributaries of 

 the subcardinal veins, and the Wolffian sinusoids exist throughout life. 

 In the rabbit the genital vein is a derivative of the mesonephric azygos. 

 The Wolffian sinusoids, except as they may form parts of large veins» 

 entirely disappear. 



The foregoing observations may be summarized in tabular form. 

 In the embryos of Torpedo, Necturus, and the rabbit alike, the veins 

 of either side are arranged thus: 



.... Mesonephric Azygos .... 



Caudal 



Space occupied by Wolffian 

 Tubules and Sinusoids 



Subcardinal 



Azygos 



1) F. HocHSTETTER, Beiträge zur Entwickelungsgeschichte des 

 Venensystems der Amnioten. Morphol. Jahrb., Bd. 20, 1903, p. 577. 



