188 OUTLINES OF CHORDATE DEVELOPMENT 



venosus, and the hepatic vein comes to open directly into it 

 instead of into the sinus venosus. 



As the pronephroi degenerate the pronephric sections of the 

 posterior cardinal veins diminish also, and by the time of 

 metamorphosis they have entirely disappeared. The ductus 

 Cuvieri consqeuently remain as the proximal parts of the 

 anterior cardinal veins only, and are sometimes known as the 

 anterior; or superior vence cavce or precaval veins. As the result 

 of these changes, all of the blood from the posterior parts of 

 the body wall, and from the tail, passes directly to the heart 

 through the median cardinal and postcaval veins (Fig. 68). 



The development of the mesonephroi, which begins as the 

 pronephroi diminish, entirely alters the relations of the median 

 cardinal vein. On each side the tubular components of the 

 mesonephros, whose development will be described below, 

 push into this vein, dividing it roughly into three parallel chan- 

 nels, one median and two lateral (Fig 68). The caudal vein 

 remains for a time, opening directly into the posterior end of 

 the median channel, while iliac veins, coming from the hind- 

 legs, open into the lateral channels. The caudal vein dis- 

 appears later, of course, while the iliac veins remain as the 

 chief vessels leading to the mesonephric region. 



The arrangement of the vessels in the adult may now be 

 understood easily. The iliac veins and lateral channels of the 

 median cardinal vein, with which they are continuous, become 

 the afferent or advehent mesonephric veins or the renal portal 

 veins (Fig. 68, C). The small veins from the posterior body 

 wall (posterior vertebral veins) open into the renal portal veins. 

 The vascular spaces of the mesonephros remain connected, 

 by a series of short pathways the revehent mesonephric or 

 renal veins, with the median channel of the median cardinal 

 vein, which therefore remains alone as the posterior continu- 

 ation of the postcaval vein. 



Summarizing we may say that the inferior vena cava or post- 

 caval vein is composed of four different elements. An hepatic 

 section derived from the left vitelline vein, is followed by a 

 short section which represents a new structure; next comes a 



