Till: EMBRYOLOGY OF THE OPOSSUM 14f 



The veins of the liver and mesonephros. In connection 

 with stages 30 and 31 I have described the conversion of the 

 original paired omphalomesenteric veins within the body into 

 a single vein starting on the left of the gut (orginal left 

 omphalomesenteric) then swinging dorsally around behind 

 the gut (dorsal anastomosis between the omphalomesenterics) 

 and finally entering the liver on the right side (original right 

 omphalomesenteric). It was also mentioned that within the 

 liver the right omphalomesenteric, though it develops numer- 

 ous branches in all directions, retains its principal channel 

 intact. Both right and left omphalomesenterics can be recog- 

 nized at their anterior extremities just before they join to 

 empty into the sinus, but posterior to this point only the right 

 can be followed continuously through the liver. 



It was also mentioned that during stage 31 the folding in of 

 the lateral body wall, which is converting the original wide 

 communication between the mid-gut and the yolk sac into 

 the narrow yolk stalk, causes the anterior third of the um- 

 bilical veins to swing vent rally and approach each other near 

 the middle line. Here they become involved in the growing 

 liver and during stage 32 are broken up into sinusoids just as 

 the hepatic portion of the left omphalomesenteric was before 

 them. Further changes in these vessels will be described in 

 connection with stages 33 and 34. 



The liver is enlarging very rapidly in stage 32 and on the 

 right side, where there is no stomach in the way, the so-called 

 right coronary appendage of the liver expands dorsad against 

 the mesonephros and the dorsal mesentery. Small vessels 

 from the liver and from the subcardinal vein invade the 

 mesentery, unite, and enlarge to form a conspicuous anasto- 

 mosis which leads directly from the subcardinal to the right 

 omphalomesenteric vein within the liver (fig. 47, C). 



At the same time another important change is taking place 

 in the posterior portion of the subcardinals. From the time 

 of their origin the subcardinals have always communicated 

 with the postcardinals by means of numerous sinusoids 

 running through the mesonephros (the renal portal system). 

 Now in stage 32 two of these anastomoses at the level shown 



