LEPIDOSIREN PARADOXA. 
117 
development, the latter vessel resting on the dorsal surface 
of the former in the adult. The development of the pos- 
terior vena cava, as just described, is very similar to the 
condition in Urodeles ( 11 ), where the new vessel connects 
the fused portions of the posterior cardinal veins (which, 
however, separate again in L ep id o siren) directly with the 
heart through the liver. 
As the lung and foregut extend backwards, they form a 
barrier between the left posterior cardinal vein and the pos- 
terior vena cava (right posterior cardinal), so that their trans- 
verse anastomoses gradually disappear from before backwards, 
even the fused portions of the posterior cardinal veins in the 
posterior mesonephric region (interrenal vein) are wedged 
apart again, and in the adult only one or two transverse 
anastomoses persist across the middle line towards the pos- 
terior end of the body (compare Text-figs. 24 e and 3). At 
the. same time the two posterior cardinal vessels lose their 
connection with the renal portal vessels at the caudal ex- 
tremity of the mesonephros, and no longer receive blood 
from the caudal vein. Finally, therefore, the main inner 
channels on the inner dorsal surfaces of the mesonephros are 
formed by the left posterior cardinal and the posterior vena 
cava (right posterior cardinal) respectively. With the 
atrophy of the pronephros the anterior part of the right 
posterior cardinal vein loses its connection with the posterior 
vena cava in the region of the pronephric glomerulus (com- 
pare Text-figs. 24 E and 3, r. P. Card), and is represented in 
the adult by a short vessel that receives one or two small 
tributaries from the body-wall and a vein from the vertebral 
region, and joins the anterior cardinal vein to form the right 
duct of Cuvier. The left posterior vein persists as a long, 
somewhat slender trunk passing forwards from the left meso- 
nephros between the intestine and the body-wall to the left 
duct of Cuvier (Text-figs. 24 e and 3, 1. P. Card.). 
Anterior Cardinal Veins.— The anterior cardinal 
veins appear early in the lateral regions of the head (Stage 
25), arching backwards dorsal to the branchial vessels, and 
