269 



tributary from the rectal gland. Anteriorly they have a large con- 

 nection with the azygos section of the cardinal, of which they are 

 generally regarded as a continuation. 



The cephalic end of the liver is arched so that its lateral borders 

 fuse with the posterior body wall near the root of the mesentery. 

 Thus a caval mesentery is formed on either side, but is rather more 

 extensive on the left. On the left side, in an embryo of 37 mm and 

 in those older, several branches pass across this mesentery from the 

 subcardinal vein to the hepatic sinusoids. In the adult Torpedo 

 HocHSTETTER has described ^) and figured 2) this important connection, 

 which he believes forms a true vena cava inferior. The connection with 

 the liver merely completes the vena cava. The bulk of that vein is 

 represented in fishes by the subcardinals, and their fusion to form the 

 genital sinus. Fig. 6, G. S., is analogous with a part of the left renal 

 vein of mammals. 



The following conclusion may be drawn from this study of the 

 Torpedo. The renal portal system is not an anastomosis between 

 Rabl's interrenal vein "which arises as a cranial prolongation of the 

 Vena caudalis" and the posterior cardinal veins. This generally ac- 

 cepted view is rendered untenable by the stage of the Torpedo shown 

 in Fig. 4, in which caudal and cardinal veins have formed a single 

 continuous vessel. A similar condition exists in Acanthias of 28 mm. 

 The renal portal system of these forms is due to the sinusoidal sub- 

 division of the posterior cardinal vein by the Wolffian tubules. 



In Bdellostoma the tubules remain simple, comparable with those 

 of the embryo Torpedo. Consequently in this genus, no renal portal 

 system has been found. According to Jackson^), the caudal vein 

 divides into the two posterior cardinal veins which pass forward along 

 the aorta receiving somatic vessels and segmentally arranged renal 

 veins. This description applies equally well to the Torpedo of Fig. 4. 



The Amphibia diifer from the Torpedo chiefly in the extensive 

 fusion of the subcardinal veins, and their early free communication with 

 the hepatic circulation. This is why the vessel in Amphibia has ge- 

 nerally been called the vena cava inferior although the corresponding 

 vessels in fishes have always been designated as the posterior cardinals. 



1) F. HocHSTETTER, Beiträge zur vergleichenden Anatomie und 

 Entwickelungsgescbichte des Venensystems der Amphibien und Fische. 

 Morphol. Jahrb., Bd. 13, 1887, p. 167. 



2) 1. c. p. 131, Fig. 3 (not Fig. 4). 



3) C. M. Jackson, An investigation of the vascular system of Bdello- 

 stoma Dombeyi. Journ. Cin. Soc. Nat. Hist., Vol. 20, 1901, p. 31. 



