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The diaphragm is a partition that separates the peritoneal cavity from the pericardial and pleural cavities. At this time it is largely composed of the septum transversum.

The pleuroperitoneal membranes expand to close the pleuroperitoneal opening and contribute to the dorsolateral part of the diaphragm.

Muscle tissue begins to differentiate in the dorsal part of the septum transversum where the phrenic nerves enter.

Source: Atlas of Human Embryos.