Intermediate mesoderm

The intermediate mesoderm is a loosely-packed cord of mesenchymal cells between paraxial and lateral mesoderm. Unlike the paraxial mesoderm, it does not form segments. It gives rise to the urinary system (mesonephric duct or ductus mesonephricus and nephrogenic cord).

Lateral mesoderm

Mesoblast cells move into cranial, lateral and caudal positions from the middle region of the primitive streak. These cells form the lateral mesoderm (lateral plate), which is a continuous, unsegmented layer between ectoderm and endoderm.
At S9, small splits arise in the cranial region of the lateral plate and cranial to the oropharyngeal membrane. They soon coalesce to form a horseshoe-like cavity (intra-embryonic coelom). This coelom opens outwards on both sides at the level of the first somites, and links with the extra-embryonic coelom. At S11, the intra-embryonic coelom has also formed in the caudal region of the lateral plate. Thus, the coelom represents a ring-like structure in the embryonic body (coelomic ring). The cranial part remains closed outwardly and the caudal part is in broad contact with the extra-embryonic coelom. The coelom is lined with coelomic epithelium. The lateral plate is subdivided by the intra-embryonic coelom into a parietal mesoderm (somatopleura-mesoderm), which lies beneath the ectoderm, and a visceral mesoderm (splanchnopleura-mesoderm), which is situated between the coelomic epithelium and the endoderm. The pericardial cavity, the two pleural cavities, and the peritoneal cavity arise from the coelomic ring. Due to the folding of the embryo, the connection from intra- to extra-embryonic coelom narrows down to become the umbilical coelom. See section Intra- and extra-embryonic cavities.