408 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. vi 



The first part of the venous system (Fig. 190, A) indeed of the 

 vascular system to take definite form consists of the two vitelline 

 veins, which pass tail wards over the anterior surface of the \>lk 

 (stage 24). Anteriorly they become conjoined to form the hi -art 

 while posteriorly they are continued into the rudiments of the vitelline 

 network (stage 24-25). The appearance of the vitelline veins is 

 followed almost immediately by the development of a longitudinal 

 venous channel on each side anteriorly, superficial to the aortic arches 

 the anterior cardinal vein. At its hinder end the anterior 

 cardinal is continued into a set of venous spaces in the region of the 

 pronephros (pronephric sinus) and onwards behind this as the 

 posterior cardinal vein. 



The pronephric sinus is continued along its outer edge, by a 

 number of channels, into the vitelline network of venous spaces, 

 lying in the splanchnic mesoderm over the surface of the yolk. In 

 this network a conspicuous channel becomes apparent, leading from 

 the anterior end of the pronephric sinus outwards' to the vitellmr 

 vein, and so, by way of the anterior part of the vitelline vein, to the 

 heart. This vessel so constituted, which at first makes a wide sweep 

 over the lateral surface of the yolk, is the Duct of Cuvier (Fig. 190, 

 b, d.C). As development goes on the Ducts of Cuvier become 

 greatly shortened and at the same time widened until eventually 

 they form in the adult very short wide channels for the conveyance 

 of the blood from the cardinal veins into the sinus venosus (see Fig. 

 190, b and c and d). 



The posterior cardinal vein on each side appears about stage 24 

 in the form of spaces along the course of the archinephric duct 

 which become joined up so as to form two longitudinal vessels 

 running parallel to the duct, one on its mediodorsal the other on its 

 ventrolateral side, the two vessels being joined round the duct by 

 numerous anastomoses (Fig. 190, b and c, p.c.v). 



These posterior cardinal veins accompany the archinephric ducts 

 throughout their length and just in front of the cloaca are joined by 

 the hind ends of the bifurcated subintestinal vein (see below) and of 

 the dorsal aorta. The vessel formed on each side by the union of 

 these three elements is continued hack past the cloaca and unites 

 with its fellow of the opposite side to form a vessel lying immediately 

 beneath the post-anal gut. This vessel is to be interpreted morpho- 

 logically as a post-aiiiil portion of the subintestinal vein and as \\ill 

 be shown later it is destined to become the caudal vein of the adult. 



It will now be convenient to trace out the subsequent fate of 

 what may be called tin- dorsal venous system, consisting primarily 

 of tin- ;interior and posterior cardinal veins. 



POSTKKIOK CARDINAL VKINS. The posterior cardinal vein was 



left in the forn; of a pair of vessels, an inner ami an outer, lying 



to the jirehinrphne duel and connected to-vtiier hy numerous 



;Hi:i-1.om08es. As the opist I lom-phros develops between ilie.se channels 



i h-y consequently come, ovei a considerable part of their length, to be 



