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. It is thought to be derived from neural crest. Neural crest cells also migrate ventrally along the neural tube in spinal regions. The prechordal plate begins to migrate laterally forming bilateral condensations caudal to the optic vesicles.
Pia mater is present at the mesencephalic level.
Vascularisation around the spinal neural tube begins. Depicted in a reconstruction of a stage 13 embryo. The prechordal plate mesenchyme has now formed the bilateral pre-mandibular condensations. 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.