Once again, scientists find wholeness new and fascinating, when wholeness is in fact the scientific truth, and always has been. The mesentery (as well as the mediastinum within the chest and meninges around the brain) forms from the coelomic bags that line the spaces which open up within the early embryo. You can see these spaces in this diagram from The Developing Human by Moore, Persaud, and Torchia:
These spaces start off as a single horseshoe-shaped circulation around the edge of the embryo, and are folded into the body in the incredible origami that is our development. It is no surprise to me, nor to any of the practitioners of visceral manipulation, who work daily with these membranes, that it is really all one piece.
People tend to forget that our human penchant for naming things creates useful distinctions, but that does not make the distinctions real or absolute. The peritoneum, mesentery, mesenteric root, and mesocolon are all parts of one organic organising membrane in which our lower digestive tract lives and has its being.