186 OUTLINES OF CHORDATE DEVELOPMENT 



The ductfts Cuvieri pass from the sinus venosus obliquely 

 upward in a nearly vertical plane, to the body wall where they 

 divide, passing thence anteriorly and posteriorly. The anterior 

 branches are the anterior cardinal veins. These continue 

 forward as the superior jugular veins, receiving blood from the 

 brain and dorsal parts of the head, and from the region of the 

 eye and ear (facial branch). The inferior jugular veins, coming 

 from the region of the mouth, sucker, and ventral surface of the 

 head, open into the roots of the ductus Cuvieri, just before 

 these open into the sinus venosus. 



The posterior branches of the ductus Cuvieri are the posterior 

 cardinal veins. These are primarily the veins of the body wall 

 and the excretory systems. They pass posteriorly through the 

 pronephric region, and thence along the medial side of the 

 pronephric ducts (see below) (Figs. 68, 72, 76) receiving blood 

 from the veins of the body wall (segmental veins). A median 

 caudal vein, passing forward through the entire length of the 

 tail, just ventral to the dorsal aorta or caudal artery, upon 

 reaching the body cavity divides above the cloacal region, 

 and its branches connect directly with the extremities of the 

 posterior cardinal veins, so that these receive blood from the 

 tail also. In the region of the head kidney or pronephros (see 

 below) each of these veins forms a large sinusoidal system 

 among the tubules of this excretory organ. Shortly after the 

 opening of the mouth, as the definitive kidney or mesonephros 

 (see below) commences to develop, the arrangement of the pos- 

 terior cardinal veins is profoundly modified. The hinder parts 

 of the two veins begin to fuse together at about 15 mm., and 

 ultimately form a median vessel, which may be termed the 

 median cardinal vein (Fig. 68). Anteriorly this vessel effects a 

 new and direct connection with the sinus venosus. 



This connection is brought about by the development of the 

 posterior or inferior vena cava or postcaval vein. This important 

 vessel is first indicated by the marking out of a definite pathway 

 in the vessels of the dorsal side of the liver, vessels which are 

 branches of the left vitello-intestinal vein (Shore). This 

 vessel then leaves the surface of the liver and passes through 



