At the end of gastrulation, the embryonic mesoderm consists of three regions, paraxial, intermediate and lateral mesoderm. As development proceeds, clefts develop within the right and left lateral mesoderm. Later these clefts coalesce, forming a cavity which splits the lateral mesoderm into an outer layer of somatic mesoderm and an inner layer of splanchnic mesoderm (Fig 10.1). The cavities between the two layers of mesoderm on the left and right sides are referred to as coelomic cavities. The left and right developing coelomic cavities located on either side of the midline extend cranially, meet and fuse in front of the developing neural and cardiogenic plates, forming a horseshoe‐shaped coelomic cavity (Fig 10.2). The lateral walls of the coelomic cavity are composed of somatic mesoderm which fuses with ectoderm, forming somatopleure. The medial walls are composed of splanchnic mesoderm which fuses with endoderm, forming splanchnopleure. The mesodermal cells lining the coelomic cavity differentiate into a simple squamous epithelium, referred to as mesothelium. Following cranial, caudal and lateral folding of the embryo, the convex region of the horseshoe‐shaped coelom occupies a position ventral to the foregut and the developing heart, and gives rise to the primordium of the pericardial cavity. The right and left limbs of the coelomic cavity are connected to the pericardial cavity by the pericardial–peritoneal canals (Fig 10.3). Lateral body folding results in division of the developing embryonic coelom into an intra‐embryonic and an extra‐embryonic region. Subsequently, the intra‐embryonic coelom gives rise to the pericardial, pleural and peritoneal cavities. The extra‐embryonic coelom is associated with the developing foetal membranes. The intra‐embryonic and extra‐embryonic coelomic cavities, which are initially continuous at the umbilicus, subsequently become separated from each other.