vi VEINS OF LEPIDOSIKEN 411 



primitive, elongated form, extending far forwards in the splanchno- 

 coele, and the front end of the right opisthonephros is in immediate 

 apposition to the tip of the liver which is also situated dorsally and 

 on the right side. It is no doubt the approximation of the tips of 

 these two organs, still persisting in the adult Dipnoan, which 

 paved the way for the establishment of direct continuity between 

 their vascular networks and the consequent short-circuiting of the 

 renal blood through the hepatic vein into the heart (Graham Kerr, 

 1910). 



The ladder-like connexions . between right and left posterior 

 cardinals in the region anterior to the inter -renal vein gradually 

 disappear in turn from before backwards (Fig. 190, d and e). 



Anterior Cardinal Veins. — Apart from relatively less important 

 details, the chief change which comes about in regard to the anterior 

 cardinal is the diversion of the blood-stream about the level of the 

 otocyst into a more laterally placed channel called the lateral 

 cephalic vein, which eventually becomes intercalated in the course 

 of the anterior cardinal, and to all appearance forms simply a portion 

 of that vein (Fig. 190, d, l.c). The anterior cardinal vein at first 

 passes back along the side of the head region (following the course 

 shown by the dotted outline in Fig. 190, d), ventral to the otocyst 

 and internal to the posterior cranial nerves, to join the front end of 

 the pronephric sinus. Presently (stage 30) a branch of the anterior 

 cardinal vein makes its appearance and extends backwards external 

 to the ganglion of the seventh cranial nerve, bending inwards and 

 rejoining the anterior cardinal between the eighth and ninth cranial 

 nerves. A little later (stage 31) the vessel forming the outer side 

 of this loop becomes prolonged back and forms a second loop external 

 to the ninth cranial nerve and rejoining the main vessel between 

 nerves IX and X. Finally (stage 31 + ) a similar extension backwards 

 occurs external to the vagus, rejoining the anterior cardinal vein 

 just behind it. The lateral cephalic vein develops from the outer 

 portions of these three vascular loops, the development of each of 

 the three segments being followed by the atrophy of the correspond- 

 ing section of the original anterior cardinal, except in the case of the 

 most posterior section which persists as a short wide vein opening 

 along with the posterior cerebral vein (ce") into the definitive anterior 

 cardinal. 



Ventral Venous System. — In addition to the dorsally placed 

 cardinal veins there exist certain important veins situated more 

 ventrally and developed in relation with the vitelline veins. The 

 vitelline veins spread backwards on each side as a wide vessel con- 

 sisting of an enlarged channel of the vitelline network which covers 

 the whole surface of the yolk. Posteriorly they unite in the mid- 

 ventral line to form a very short subintestinal vein in front of the 

 anus (Fig. 190, C, s.i.v). Later on the space bounded by the two 

 vitelline veins becomes bisected by a median ventral vein which 

 looks like a prolongation forwards of the subintestinal vein and 



