THE EMBRYOLOGY OF THE OPOSSUM 165 



The abdominal, epigastric and umbilical veins. With the 

 collapse of the allantois in stage 34 retrogressive changes be- 

 come noticable in the umbilical veins, so that this seems to 

 be a good point at which to review their history and comment 

 on their significance. 



In stage 24 when angiogenesis begins, one of the first vessels 

 to appear in the body is a vein running along the lateral body 

 wall. This vein, which has been described as the umbilical, 

 is at first the vein of the anterior limb ridge. In later stages 

 it grows caudad through the lateral body wall until in stages 

 28 and 29 it reaches the level of the developing posterior limb 

 ridge, which it then invades. At this time the postcardinal 

 has also reached this level and the two form numerous anas- 

 tomoses within the posterior limb ridge. Simultaneously 

 another complication is introduced by the developing allantois, 

 which becomes involved in this umbilical-postcardinal plexus. 

 As the allantois enlarges and projects beyond the body wall 

 in stages 29 and 30, it carries its portion of the plexus with it, 

 and is thus connected with both the umbilical and postcardinal 

 veins. 



In stage 30 the rounding up of the body wall carries the 

 umbilical vein further ventrad, and causes its direct connec- 

 tion with posterior limb ridge to become reduced and eventu- 

 ally lost. Its connection with the anterior limb ridge (the 

 radial vein) is preserved for a longer time, but also eventually 

 (stage 31) disappears. The connection with the allantois, 

 however (and thus indirectly with the hindlimb bud through 

 the postcardinal-allantoic plexus), is strengthened, and from 

 this time on the umbilical vein is devoted almost exclusively 

 to the transfer of blood from the allantois to the liver, and is 

 homologous with the umbilical or placental vein of higher 

 mammals. 



The connection between the allantois and the postcardinal 

 is retained at first as a plexus of small veins in the middle of 

 the ventral abdominal wall, running caudad and joining the 

 postcardinal at and around the mouth of the external iliac 

 vein. Later (stages 33 and 34) this plexus is reduced to two 



