410 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



separated from one another by a considerable space in, which the 

 renal organ lies. As this happens the inner components of the two 

 posterior cardinals become approximated and eventually undergo 

 fusion with one another to form an inter-renal vein (Fig. 190, d, ir.v). 

 In Lepidosiren this fusion is only temporary and the two compon- 

 ents again recede from one another — remaining, however, connected 

 by a small number of anastomotic vessels (Fig. 190, e). There now 

 takes place a severance of the continuity of the inner component at 

 its hinder end (Fig. 190, e,*), and a little later a similar severance of 

 the outer component at its front end. The physiological result of these 

 interruptions of continuity is that the blood from the caudal region 

 now reaches the opisthonephros entirely by way of the outer com- 

 ponent, it then passes through the substance of the kidney and is 

 drained away entirely by the inner component. In other words the 

 outer component and its backward continuation has now become the 

 renal portal vein. 



While these changes are going on in the opisthonephric region of 

 the posterior cardinal the portion of that vein in front of the opistho- 

 nephros takes on the form of a single channel, the original inner 

 component becoming enlarged while the outer component becomes 

 reduced and eventually disappears. The right and left posteriorcardinal 

 veins in this region in front of the opisthonephros become connected 

 by numerous transverse vessels (Fig. 190, d). A short circuit now 

 becomes established by which the blood from the right posterior 

 cardinal can pass direct to the sinus venosus through the substance 

 of the liver (Fig. 190, d, p.v.c). 



This short-circuiting channel is of great morphological import- 

 ance. It constitutes the intrahepatic or headward section of the 

 posterior (" inferior ") vena cava, which in the higher vertebrates 

 becomes the largest vein in the body. Its appearance here is followed 

 by two important results. (1) The main blood-stream from the 

 kidney region tends to pass to the heart, more and more by this 

 direct channel, which in correlation* with this becomes larger and 

 larger. (2) The portion of right posterior cardinal vein lying behind 

 its junction with the intrahepatic channel becomes correspondingly 

 enlarged. 



These two components together constitute the definitive posterior 

 vena cava, in which vessel therefore we recognize two fundamentally 

 distinct portions, an anterior or intrahepatic, and a posterior or 

 cardinal. A secondary result of the diversion of the blood-stream 

 from the right posterior cardinal vein through the hepatic com- 

 ponent is that the anterior portion of the former vein, lying in front 

 of the junction of the two components, becomes relatively reduced in 

 size. It loses its continuitytwith the rest of the right posterior 

 cardinal and persists as the small vein shown at v in Fig. 190, e. 



An important clue to the origin of the posterior vena cava in 

 phylogeny is given by the condition seen in the adults of existing 

 Lung-fishes. The opisthonephros in these vertebrates retains its 



