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The anterior cardinal veins (a.cd. Fig. 1 A, s.j. Fig. 1 C) develop 

 early as the anterior roots of the ductus Cuvieri. They extend ven- 

 trally to the auditory capsule, forward to the sides of the brain and 

 finally reach the olfactory region. These veins become the superior 

 jugular veins and seem to be equivalent to the external jugular veins 

 of these forms. The inferior jugular veins {i.j. Fig. 1 C) develop late 

 and precisely as in Rana, for example. The arrangement and mode 

 of development of the posterior cardinal veins (p.cd. Fig. 1) are 

 typical and need no further mention. There should be mentioned, 

 however, one important resemblance to the Amphibia, namely, the 

 breaking up of the posterior cardinal veins into capillary networks 

 among the tubules of the pronephros. This is totally unlike the 

 Elasmobranch arrangement where the cardinal veins are not especially 

 related with the pronephros, but simply expand into wide sinuses 

 before entering the heart. 



The caudal vein (c. Fig. 1) appears first as an isolated vessel 

 beneath the caudal aorta and passes some distance anterior to the 

 cloacal region before bifurcating. Later its branches anastomose with 

 the posterior cardinal veins, and as the mesonephros develops the 

 anastomoses become coextensive with it and increased in number. The 

 most posterior of these anastomoses is the largest and anteriorly from 

 that the branches of the caudal vein remain present irregularly as the 

 paired interrenal veins (i.r. Fig. 1 A). The formation of the important 

 afferent renal vessels described as the posterior trunk veins (tr. Fig. 1 C) 

 results from the fact that the caudal vein anastomoses with the 

 posterior cardinals some distance anterior to their extremities, which 

 then remain as the posterior trunk veins and contribute largely to the 

 renal-portal system. 



Interest in the venous system of Ceratodus probably centres in 

 the inferior vena cava, for there are no forms below the Dipnoi where 

 this important trunk is represented. In Ceratodus the vena cava is 

 formed in two sections. Its posterior section is the hinder portion of 

 the right posterior cardinal vein {p.cd., v.c. Fig. 1) which anteriorly 

 has effected a new connection with the anterior section of the vena 

 cava. This latter is an independently formed vessel {v.c. Fig. 2 A) 

 appearing late — twenty-eight days after hatching, and rapidly growing 

 backward through the liver and connecting with the right posterior 

 cardinal vein (Fig. 2B) at about the time the pronephros loses its 

 functional value. After this new connection is established the anterior 

 portion of the right posterior cardinal remains as the right vertebral 



