mes·o·the·li·um 
(mĕz′ə-thēlē-əm, mĕs′-)
(mĕz′ə-thēlē-əm, mĕs′-)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.
[MESO- + (EPI)THELIUM.]
mes′o·theli·al adj.