Meninges

From S11, mesenchyme and blood vessels surround the brain and spinal cord. The mesenchyme comes from different sources: prechordal plate, unsegmented paraxial mesoderm, segmented paraxial mesoderm (somites) and the neural crest. At S13, loose mesenchyme surrounds the whole brain; at S15, it also surrounds the spinal cord. It is called primary meninx (meninx primitiva). As development progresses, the meninx subdivides into the pachymeninx (dura mater) and the leptomeninx (arachnoid and pia mater). The pachymeninx or dura mater forms a dense layer with chondrogenic and osteogenic properties (periosteal layer or external layer of the dura mater). Meningeal blood vessels differentiate into arteries and veins on its internal surface. From S18-20, the internal layer or meningeal layer of the dura mater thickens and begins to separate from the leptomeninx. By S23, the leptomeninx and the dura mater can be clearly distinguished in the cephalic region. In the region of the spinal cord, the dura mater completely lines the vertebral canal.
The cells of the leptomeninx form a continuous membrane, adjacent to the dura mater and a trabecular system which subdivides the subarachnoid space. The internal layer of the leptomeminx, the pia mater, directly covers the brain surface and the spinal cord.
At the level of the skull, the external (i.e. periostium) and internal layers of the dura mater fuse together. The layers are only  separate where the venous network joins into the large venous conduits of the dura mater. At the level of the spinal cord, the epidural venous plexus is preserved. The periostial and meningeal layers of the dura mater remain separate and form the epidural space. Thus, epidural haemorrhages occur at the level of the skull between the periostium and the bone, and at the level of the spinal column between the meningeal and periostial layers of the dura mater.