416 EMBRYOLOGY OF THE LOWER VERTEBRATES < 11. 



left duct of Cnvier 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. 



CROSSOPTERYGII. 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. 



o.c.u. 



B. 



p.c.u 

 r.v.u ....{ 



an 



a.c.v. 



S.I.U. 



s.i.is. 



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

 ( After Goette, 1890.) 



anterior cardinal vein; (in, anastomotic vein ; </.r, duct of Cuviei ; k.r. hepatic veins; //, 

 outline of liver; /./-./, left vitelline vein; 'ji.i-.r, posterior cardinal vein; /././, i i^lit vitelline \rin; 

 likf, Mihint'-stinal vein. Portions of the venous trunks which disappear during ontogeny an- slin\\n 

 by <iotted outlines ; the outline of the liver is shown in H hy an intci-nipted line. 



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

 to form the heart. Posteriorly the subintestinal vein hi furcates 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, \vi<lc communications pass between the suhiniesl.inal 

 vein and tin- posterior cardinals. Tin- ultimate fate of the suh- 

 inlestinal vein dilfers in its j.rc-cloacal and post-cloaca 1 portions. 

 Tin- tonni-r 10868 it- identity in that, it becomes nitnvly resolved 

 into portions of iht- \iirllinr nelworU. The ]>osl -eloacal portion 

 becoin*-- Converted into tin- caudal vein in the normal fashion as 

 already dt-scril't-d I'nr /., /n'(/<mirrn. It has already IMMMI iiKMitionetl 

 that wide eomniiinic.il loim weiv established betwrcn the paired 

 .1 portion of the siihintestiiial \ein and ihr posterior cardinals. 



With the breaking up of the pre-oloaoal pan of the vein the main 



