vi VENOUS SYSTEM 415 



the duct of Cuvier which does not take the wide sweep over the 

 surface of the yolk that it does in Zepidosiren during early stages. 

 This difference is related to the fact that here the first rudiment of 

 the duct of Cuvier opens into a portion of the vitelline vein which has 

 fused with its fellow to form the hind end of the cardiac tube, while in 

 Zepidosiren it opens much farther back, the result being that in Zepido- 

 siren a considerable stretch of free vitelline vein becomes incorporated 

 in the definitive duct of Cuvier (compare Figs. 191, C and 190, b). 



As in Zepidosiren the caudal vein becomes continuous with the 

 posterior cardinals and loses its continuity with the pre-anal portion 

 of the subintestinal vein. The inter-renal vein however develops 

 here simply as a forward extension of the caudal vein according to 

 Eabl. A number of anastomotic vessels connect up the inter-renal 

 vein with the " posterior cardinal " — the equivalent of the external 

 component of this vein in Zepidosiren. The posterior cardinal now 

 becomes obliterated behind the anterior one of these anastomotic vessels 

 while the inter-renal becomes separated off from the caudal vein so 

 that the whole blood-stream from the latter has to pass through the 

 kidneys to reach the inter-renal. The latter vein splits into a pair of 

 vessels eventually, thereby revealing that the inter-renal vein here is 

 homologous with that of Zepidosiren in spite of its different — no 

 doubt secondarily modified — mode of development. 



The anterior cardinal vein here again becomes in part replaced 

 by a lateral cephalic vein. 



In the lower vertebrates in general we may recognize the same 

 main trunks as occur in Dipnoi and Elasmobranchs, with differences 

 in detail. The following account gives an outline sketch of the 

 development of the venous system in the various groups, the outline 

 being filled in more fully in the case of Polypterus on account of the 

 very archaic character of this fish. 



Cyclostomata. — In the Lamprey, according to Goette (1890), 

 the pair of vitelline veins appear first, spreading backwards on either 

 side of the liver rudiment and meeting behind in the unpaired and 

 much dilated subintestinal vein (Fig. 192, A). The vitelline veins 

 break up into a network in the liver but on the left side the post- 

 hepatic section of vitelline vein disappears, so that the hepatic portal 

 vein is formed by the subintestinal and right vitelline vein (Fig. 

 192, B and C) — somewhat as in the Elasmobranch, and unlike 

 Zepidosiren where it is the right vein which disappears. The vein 

 of the " spiral valve " of the intestine arises comparatively late, at 

 the time -of metamorphosis according to Goette,. and on the opposite 

 side of the gut from that on which the subintestinal vein lies. This 

 latter is no longer ventral but high up on the right side, owing to a 

 rotation which the gut has undergone. 



The anterior and posterior cardinal veins present the peculiarity 

 that they open at first separately into the vitelline veins. Later 

 they become fused together to form the duct of Cuvier. Eventually the 



