From the 9th to 12th week of development, the pleural cavities ex-id rapidly to accommodate the reply growing lungs. By a burrowing ac-the enlarging pleural cavities invade the body wall and force a layer of mesenchyme to split off medially. This mesenchyme is added to the phragm as a peripheral rim lateral to pleuroperitoneal membranes. myoblasts originating in.the body wall ate into this mesenchymal rim and ibute to the formation of the mus-of the most peripheral part of the daighram.