138 EDWARD MCCRADY, JR. 



right side of the liver (fig. 45, central reconstruction). On the 

 other hand, it is the right side of the posterior loop which has 

 been reduced to a vestige, as seen in the cross-cut portion of 

 the central reconstruction; and this will soon be likewise lost. 

 Though the process is not quite complete, it is obvious that 

 within the body the two omphalomesenteric veins are being 

 converted into a single large vessel entering the body on the 

 left side of the duodenum and spiralling around behind it to 

 pass into the liver on the right. Within the liver the right 

 omphalomesenteric is not completely destroyed or replaced 

 by sinusoids as in placental mammals, but maintains its open 

 channel right through to the sinus venosus up until the time 

 of birth. It thus comes to play a part in the formation of the 

 postcava which will be described in the next chapter. 



The enlargement of the liver and the constriction of the 

 connection between the mid-gut and the yolk sac have caused 

 important changes in the umbilical veins. These will be dis- 

 cussed in more detail in the next chapter, but it should be 

 mentioned here that the liver has grown around the anterior 

 ends of the umbilicals (fig. 45, reconstruction) and the pos- 

 terior ends are being drawn toward the midventral line 

 (fig. 45, D). 



In the mesonephros many branches from the postcardinal 

 ramify among the tubules. In stage 31 these branches develop 

 longitudinal anastomoses along the ventral medial border of 

 the mesonephros. The new longitudinal channels are the 

 subcardinals (fig. 45, D). 



The primitive radial vein, which drained the ventral border 

 of the anterior limb into the umbilical, seems to be finally lost 

 in stage 31. In previous stages its mouth had migrated an- 

 teriorly along the umbilical until in stage 30 it emptied into 

 the angle between this vein and the duct of Cuvier. It is 

 possible that this same process of migration has now carried 

 it up the duct of Cuvier to the cardinal anastomosis or prom- 

 ontory, in which case it is preserved as the cephalic vein of 



Fig. 45 Reconstructions and sections of stage 31. The central reconstruction 

 and I and J are from 17146. A and B are from 17147. The sections are from 

 16163. G shows the real size of the vesicle. 



