Last updated 30 March 2006

• The cranial rim of the disc folds under the cephalic neural plate, wedging the septum transversum between the heart region and the yolk sac, and placing the cardiogenic area and the buccopharyngeal membrane in the future thoracic and mouth regions, respectively.

• The caudal rim of the embryonic disc contains:
1. The cloacal membrane
2. The connecting stalk, which itself contains the slender allantois.

• Caudal folding brings the cloacal membrane onto the ventral surface of the embryo, with the embryo rotating ventrally until the connective stalk lies against the neck of the yolk sac.

• Thus, ectoderm covers the whole of the embryo except for the closing neuropores and the ubilicus, from which the connecting stalk and yolk sac neck emerge.

• Fusion of the lateral edges of the endoderm creates the gut tube, with blind ends in the cranial and caudal regions forming the foregut and hindgut respectively.
• The midgut is temporarily open to the constricting yolk sac neck, which will narrow into the slender vitelline duct by the end of the sixth week.
• The buccopharyngeal membrane, capping the foregut, ruptures at the end of the fourth week, connecting the oral cavity to the pharynx; whilst the cloacal membrane, capping the hidgut, breaks down in the seventh week, forming the anal and urogenital orifices.

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The intraembryonic coelom

• The serosal (mesothelial) membranes develop to line the body cavity:
  • Parietal serosa develops from the somatopleuric lateral plate mesoderm
  • Visceral serosa develops from the splanchnopleuric lateral plate mesoderm
• The ventrolateral body wall of the embryo is called the embryonic somatopleure whilst the gut wall of the embryo is called the embryonic splanchnopleure.

• The mesenchyme connecting the gut tube dorsally to the body wall in the abdominal region disperses, and the overlying serosal layer meets dorsal to the gut tube to become the double layed dorsal mesentery, suspending the gut tube in the abdominal cavity.
• Visceral organs are classified in relation to the body wall as follows:
  1. Organs suspended in the coelom by a mesentery are intraperitoneal
  2. Organs that develop in the body wall are retroperitoneal
  3. Organs initially suspended in mesentery that become attached to the body wall are secondarily peritoneal.

• These 2 cavities communicate through a pair of dorsolateral canals called the pericardioperitoneal canals, dorsal to the septum transversum on either side of the foregut mesenchyme.

• The folding and differential growth of the embryo causes the septum tranversum to descend from the cervical region to the level of the future diaphragm, carrying with it its cervical innervation (C3, C4, C5) to form the phrenic nerve.

• During the fifth week, two coronal pleuropericardial folds develop in the lateral walls of the primitive pericardial cavity, growing medially to partition the primitive pericardial cavity into:
1. The definitive pericardial cavity ventrally, containing the heart
2. A pair of dorsolateral pleural cavities that will house the lungs, and are still continuous with the peritoneal cavity through the pericardioperitoneal canals.

• The bases of the pleuropericardial folds migrate ventrally around the body wall so that the heart becomes enveloped by the pleuropericardial folds, forming the pericardial sac.

• During the fifth and sixth weeks, 2 pleuroperitoneal membranes grow from the walls of the pericardioperitoneal canals to fuse with the septum transversum, separating the pleural cavities from the peritoneal cavity, completing partitioning of the intraembryonic coelom.

• Thus, the diaphragm has 4 components:
1. The muscle sheets of the pleuroperitoneal membranes
2. The central tendon, derived from the septum transversum
3. The right and left crura which develop from oesophageal mesenchyme
4. A peripheral ring of muscle from the body wall mesenchyme adjacent to the developing diaphragm, innervated by spinal nerves T7-T12.