1 The diencephalon develops from the median portion of the prosencephalon and consists of a roof plate, 2 alar plates, and the third ventricle.2  There is a question of whether it has a floor or basal plate. THE ROOF PLATE consists of a single layer of ependymal cells covered by vascular mesenchyme (meninges). 2  The 2 layers later combine to form the choroid plexus of the third ventricle, which closes it from above.