• At the end of week 3, spaces in the lateral plate and cardiogenic mesenchyme fuse to form a horseshoe-shaped intra-embryonic coelom (same process of development as the extra-embryonic coelom). Amniotic fluid can then circulate in this area, as it communicates with the extra-embryonic coelom.

• The curve of this horseshoe will form the pericardial cavity and the limbs will form the future pleuroperitoneal cavities.

• In the fourth week, the horseshoe will change to a pericardial cavity, with two symmetrical pericardioperitoneal canals, leading to the large peritoneal cavity.
• The future body wall is formed on the dorsal surface of the cavity of somatic mesoderm from the lateral plate, mesothelium, and surface ectoderm. The future gut wall is formed from lateral plate mesoderm, mesothelium and endoderm on the ventral surface of the intra-embryonic coelom. During this week, the embryo will fold laterally, so the outer somatopleure envelops the inner splanchnopleure.
• The three body cavities (pericardial, pleural, peritoneal) will form definitively in the second month.

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