These cells are developed from the Endothelium and mesothelium.
But very early this tissue is increased to form a massive layer by direct development from the mesothelium.
This mesodermic epithelium lining the cœlom is called the mesothelium.
The mesenchymal tissue between the mesothelium and the ectoderm forms the mass of the skeletal, muscular and vascular systems.
Again, the function of this mesothelium apart from the germ-cell is essentially excretory and phagocytic.
mesothelium mes·o·the·li·um (měz'ə-thē'lē-əm, měs'-)
n. pl. mes·o·the·li·a (-lē-ə)
The layer of flat cells of mesodermal origin that lines the embryonic body cavity and gives rise to the squamous cells of the peritoneum, pericardium, and pleura.
A layer of flattened epithelial cells that lines the membranes of closed body cavities, including the pericardium, pleurae, and peritoneum. Compare endothelium.