The first signs of the future neural tissue are apparent, and the proliferating prechordal plate is a mass of mesenchyme situated rostral to the future notochord. Paraxial mesenchyme derived from the primitive streak is also accumulating in the future head area.
According to O'Rahilly R., Müller F., (1986) the presence of pia mater is indicated around future medulla oblongata. It is thought to be derived from neural crest. Neural crest cells also migrate ventrally along the neural tube in spinal regions. Sensenig E.C., (1951)
The prechordal plate begins to migrate laterally forming bilateral condensations caudal to the optic vesicles. Pia mater is present at the mesencephalic level. O'Rahilly R., Müller F., (1986). Vascularisation around the spinal neural tube begins. Sensenig E.C., (1951)
Blood vessels may be found between the surface ectoderm and brain wall. In the occipital area a layer of cells (cellular sheath) appears around the notochord. Depicted in a reconstruction of a stage 13 embryo. O'Rahilly R., Müller F., (1986)Fig. 2B. The prechordal plate mesenchyme has now formed the bilateral pre-mandibular condensations.
Blood supply to the brain is increasing and in the midbrain, vessels penetrate the brain wall. Mesenchyme surrounds the telencephalon and penetrates the mesencephalic flexure where it is considered to be the primordium of the tentorium cerebelli. This mesenchyme is thought to be a continuation of the cellular notochordal sheath which by this stage extends to the tip of the notochord and appears to extend into the medial part of the tentorium cerebelli. The remnant of the prechordal plate- a median bridge of mesenchyme that connects the bilateral pre-mandibular condensations is also thought to contribute to the tentorium cerebelli. O'Rahilly R., Müller F., (1986).
A loose covering of mesenchyme around most of the brain has been termed the primary meninx. O'Rahilly R., Müller F., (1986). Loose mesenchyme between the vertebrae and spinal neural tube is considered to be primary meninx. Sensenig E.C., (1951)
The cellular sheath of the notochord continues to contribute to the formation of the medial part of the future tentorium cerebelli. O'Rahilly R., Müller F., (1986).
Mesenchyme may be found within the interhemispheric fissure- an indication of the future falx cerebri. According to O'Rahilly R., Müller F., (1986) a 'dural limiting layer' begins to form in basal areas of the brain, depicted in their reconstruction of a Stage 17 embryo, Fig. 3. Cranial nerves 3, 4, 5 and 12 pass through what is thought to be 'pori durales'. The dural limiting layer lateral to the diencephalon will contribute to the rostrolateral part of the tentorium. The primary meninx internal to the dural limiting layer is the precursor of the arachnoid membrane and the subarachnoid space, termed the leptomeninx, while external to the dural layer, the mesenchyme contributes to the bulk of the dura mater and is called pachymeninx. The pia mater is considered present when a cellular layer separates the blood vessels adjacent to the surface of the brain and the brain wall. By this stage it is complete. The mesenchymal condensations of the cranium are developing.
O'Rahilly R., Müller F., (1986)have described the formation of a 'meshwork' within the primary meninx and suggested that this may be due to an accumulation of fluid within the mesenchyme. An area in the roof of the fourth ventricle is very thin (the area membranacea superior of Weed) and may allow the passage of fluid through it. Cavitation of the primary meninx also takes place in spinal regions. Sensenig E.C., (1951)
The mesh-like leptomeninx is looser when compared to the pachymeninx. The pachymeninx contains veins which may be regarded as future dural sinuses. The mesenchyme adjacent to the area membranacea superior in the roof of the rhombencephalon has also become mesh-like although no dural limiting layer is present in this area.
The area of the future cisterna may be identified at the base of the forebrain between the olfactory bulb and the junction of the optic nerves to the chiasma. The skeletogenous layer may be contributing to the future falx cerebri in the hemispheric fissure. O'Rahilly R., Müller F., (1986).