The developing gut tube is attached to the anterior and posterior walls of the abdominal cavity by a double layer of peritoneum called mesentery. This double layer encloses the gut tube and hence the organs developing from it.
Anterior to the developing gut tube is the ventral mesentery. Posterior to it is the dorsal mesentery.
The dorsal mesentery extends like a curtain from the diaphragm to the base of the abdominal cavity, hence posterior to the gut tube there is no communication between the left and right halves of the abdominal cavity.
The ventral mesentery extends from the diaphragm down into the abdominal cavity, but ends with a free edge just inferior to the location of the pyloric sphincter of the stomach. Below this free edge, there is no mesentery anterior to the gut tube and therefore the left and right halves of the abdominal cavity are in direct communication.
Understanding of this fundamental arrangement of mesentery allows the development of the complex adult arrangement of mesentery to be followed.