Condition of the folding at the end of the 4th week
For a better understanding of the folding at the end of the fourth week (stage 12) several regions have to be looked at:
- The region above the umbilical cord (Fig. 9, section B)
- The region in the area of the umbilical cord (Fig. 9, section C)
- The region below the umbilical cord (Fig. 9, section D)
In the region above the umbilical cord (Figs. 10 - 11) the liver anlage (ventral) (stage 11) is formed from the endoderm and somewhat later the dorsal pancreas anlage (stage 12).
The liver diverticulum appears cranially to the umbilical vesicle (yolk sac). Liver cell bands penetrate into the mesoderm of the septum transversum. This is formed at the ventral location between the liver and cardiac anlage and contains the umbilical vein.
In the region below the umbilical cord (Fig. 13), the ventral abdominal wall closes up, whereby the primitive gut remains connected to it by the ventral mesenterium. After resorption of the ventral mesenterium, the gut is attached to the dorsal torso wall solely through the dorsal mesenterium.