Mesodermal cells have extended to cover the entire embryonic disc except at the oropharyngeal and the cloacal membranes. The disc now consists of three germ layers: ectoderm, mesoderm and endoderm. The initially homogeneous mesodermal layer soon becomes subdivided into paraxial, intermediate and lateral mesoderm. The mesodermal cells which have moved cranially to the prechordal plate contribute to the formation of the heart primordium (cardiogenic mesoderm).

The intra-embryonic coelom

Changes taking place in the lateral plate mesoderm are very important for the development of the intra-embryonic cavities. Spaces appear in the lateral plate and merge to form the intra-embryonic coelom. The dorsal and ventral linings of the cavity are called the parietal and visceral mesoderm respectively. At the borders between coelomic fluid and visceral mesoderm, and between coelomic fluid and parietal mesoderm, mesodermal cells differentiate to mesothelium. The coelom extends as a ring all around the whole embryonic disc (coelomic ring). The horseshoe-shaped cranial part remains closed to the outside. It continues in the region of the coelomic opening into the caudal part, which opens outwards to the extra-embryonic coelom (Implantation and primitive streak). The pericardial cavity arises from the medio-cranial part of the horseshoe-shaped coelom, and the paired anlage of the pleural cavities arises from the paired lateral parts. The caudal part of the coelom is largely in contact with the extra-embryonic coelom. It becomes the peritoneal cavity.
The terms somatopleure and splanchnopleure are used in different ways. Somatopleure means either ectoderm plus parietal mesoderm, or parietal mesoderm only. Splanchnopleure means either endoderm plus visceral mesoderm, or visceral mesoderm only.