Double membrane in the abdomen. Visceral peritoneum covers some organs, while parietal peritoneum covers the inside of the abdominal wall. Where peritoneum overlaps with other peritoneum, mesenteries and the omenta are formed.
Consider the peritoneum to be a balloon. The gut develops just outside it, and is pushed into it, invaginating one side of the balloon into the notional cavity, producing a double layer and then stretching that double layer as th gut grows. The outside layer sticks to the inside of the body cavity, the inside layer sticks to the outside of the gut, the gap closes on itself in some places, and is dragged out as a double-sided sheet - the mesentery and the plumbing and wiring passes within that sheet, and more directly through the larger gaps where the peritoneum doesn't close itself about the gut at the back of (part of) the abdomen.
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