416 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



left duct of Cuvier disappears completely (Fig. 192, C) the blood from 

 the left cardinals passing to the right side by a new anastomotic 

 vessel which develops ventral to the dorsal aorta (Fig. 192, C, an) — ■ 

 an arrangement presenting a remarkable analogy with what happens 

 in certain Mammals. 



Crossopterygil— In these archaic Teleostomes the main features 

 of the development' of the venous system' have been investigated in 

 Polypterus — the less specialized of the two surviving genera (Graham 

 Kerr, 1907). 



In the earliest stage described there is a well-developed sub- 

 intestinal vein which in front breaks up into a vitelline network. 



a.c.v. 



s.i.ix 



an 



ax.v. 



pcu 



uu 



7TUU 



S.l.V. 



Fig. 192. — Development of veins in Petromyzon as seen from the ventral side. 

 (After Goette, 1890.) 



a.c.y, anterior cardinal vein; an, anastomotic vein ; d.C, dnct of Cnvier ; Kv, hepatic veins; li, 

 outline of liver; tv.v, left vitelline vein; p.c.v, posterior cardinal vein; r.v.v, right vitelline vein; 

 s.i.v, subintestinal vein. Portions of the venous trunks which disappear during ontogeny are shown 

 by dotted outlines ; the outline of the liver is shown in B by an interrupted line. 



This drains into a pair of lateral vitelline veins which unite in front 

 to form the heart. Posteriorly the subintestinal vein bifurcates to 

 pass on each side of the cloaca and then joins again to form the 

 post-anal subintestinal vein. In its double cloacal portion, and also 

 in front of this, wide communications pass between the subintestinal 

 vein and the posterior cardinals. The ultimate fate of the sub- 

 intestinal vein differs in its pre-cloacal and post-cloacal portions. 

 The former loses its identity in that it becomes entirely resolved 

 into portions of the vitelline network. The post-cloacal portion 

 becomes converted into the caudal vein in the normal fashion as 

 already described for Lepidosiren. It has already been mentioned 

 that wide communications were established between the paired 

 cloacal portion of the subintestinal vein and the posterior cardinals. 

 With the breaking up of the pre-cloacal part of the vein the main 



