1. The umbilical (abdominal) vein has lost its direct connection with the sinus venosus. The proximal segment, cephalad of the liver, has disappeared, and its blood now passes directly into the hepatic circulation by its union with the portal vein.
(Cf. stage schema [Figs. 251] and [252].)
2. The postcaval vein has made its appearance, largely replacing the posterior cardinal veins, whose proximal segments became converted into secondary vessels (azygos) uniting the system of the postcava with that of the duct of Cuvier (mammalian præcava), while their distal segments are transformed into the distal portion of the postcava.
The postcava, therefore, is made up of two districts:
(a) The proximal portion is a new vessel, developed in connection with the hepatic venous system.
(b) The distal portion is derived from the distal segments of the original posterior cardinal veins.
The termination of the hepatic veins in the postcava corresponds to the stage shown in schema [Fig. 256].
[Fig. 263] gives a schematic representation of the arrangement of the venous system in a typical urodele amphibian (Salamandra maculosa).
In Fig. 264 the dissected venous system of Necturus maculatus, the mud puppy, is shown in an injected preparation.