As described above, the pleu-roperitoneal membranes close the openings of pleuroperitoneal canals that connect the pleura! and peritoneal cavities. As the pleuroperitoneal membranes develop, the pleuroperitoneal openings become reduced during the 7th week of development and lose completely during the early part of the 8th week. As mentioned above, the myoblasts originating in the septum transversum migrate into the pleuroperitoneal membranes and the bulk of diaphragm muscle develops in these membranes.