The MorpJwlogy of the Tadpole of Xenopus laevis. 255 
one of the posterior cardinals to a post-caval is still more marked. 
Further, in Amphioxus the sub-intestinal vein is really a plexus of blood- 
vessels ; in Myxinidae the portal continuation of the internal intestinal 
receives " the genital vein and some veins from the anterior part of 
the body" (Sedgwick); "in different Teleostomi it may also receive 
the veins from the pyloric coeca, from a portion of the air-bladder, the 
gonads, and, as previously mentioned, a tributary from the caudal vein " 
(Bridge). Various veins, which usually open into the posterior cardinals, 
may therefore open into the hepatic portals, i.e. into a portion of the sub- 
intestinal. 
Taking these facts into consideration, I cannot but express the belief 
that the posterior cardinals are not (as Bridge assumes) ''new" forma- 
tions, but represent, together with the internal intestinal and hepatic 
portal, the sub-intestinal plexus of Amphioxus. The caudal should be a 
new development. 
In the Xenopus tadpole the sub-intestinal plexus is represented by the 
following vessels : — 
1. A vein running from the posterior end of the abdomen, through 
the intestines, from which it receives numerous branches, and opening 
into the postcaval immediately after the latter leaves the mesonephros. 
It probably represents the internal intestinal of fishes, and since it drains 
the same organs as does the anterior abdominal of the adult (rectum and 
small intestine) it will probably later on become attached to the ventral 
body-wall, develop somatopleuric veins in connection therewith, and 
so become the anterior abdominal. 
2. Two dorso-lateral splanchnopleuric veins, one opening into the 
internal intestinal, the other into the hepatic portal ; these cannot 
represent the lateral veins of fishes since the latter are somato- 
pleuric ; their attachments are against their being regarded as 
posterior cardinals, but if the sub-intestinal is taken to be a plexus, 
then the points of attachment of particular veins would not matter 
very much (consider, e.g. the various attachments of the spermatic veins 
in fishes), and we may consider these two veins as the homologues of the 
posterior cardinals of fishes. The one opening into the hepatic portal can 
be clearly recognized in the adult (Sp Fig. 5, Dreyer, already quoted) for 
in both it passes through the pancreas. That the other represents the 
mesenteric of the adult (Mes Fig. 5, Dreyer), is not certain, but is never- 
theless probable, since in both cases there are only three posterior 
splanchnopleuric veins in connection with the hepatic portal. 
3. On leaving the kidney the postcaval divides into two, both 
branches going to the right side of the body into the liver, the one branch 
remains on the dorsal aspect of the liver, receives numerous hepatic veins, 
passes in almost a straight line to the sinus venosus, and represents the- 
