The alar plates (side walls and floor of the diencephalon)
From the 7th week (stages 16-19) a thickening of the diencephalon walls occurs as well as the formation of three copious swellings that protrude into the IIIrd ventricle. Dorsally to ventrally, these are the epithalamus, the thalamus and the hypothalamus. In 70% of individuals the thalami adhere to one another in the median plane (interthalmic adhesion), whereby the IIIrd ventricle receives a ring-shaped form.
The epithalamus (stage 16) develops out of the roof and the upper portions of the alar plates. As was already mentioned, the epiphysis (stage 15) and the choroid plexus of the IIIrd ventricle, which is composed of the lamina epithelialis and the tela choroidea, belong to it. The epithalamus is delimited form the thalamus by the dorsal diencephalic sulcus (stage 17). Sometimes, in humans, a diverticulum, the paraphysis, arises above the foramina of Monro that in lower vertebrates is always present. The habenular nuclei arise laterally in the epithalamus that are assigned to the limbic and olfactory systems and play an important role in the release of emotional reactions as well in steering behavior. There are also two small transversal connections, the habenular commissure and the posterior commissure (that already belong to the midbrain). The massive growth of the thalamus leads to a disappearance of the dorsal diencephalic sulcus and to a caudal relocation of the elements of the epithalamus
The hypothalamic sulcus
The hypothalamic sulcus divides the alar plate into dorsal and ventral regions, from which the thalamus and the hypothalamus emerge. The hypothalamic sulcus thus differs fundamentally from the sulcus limitans that runs between the basal plate and the alar plate!
In contrast with the epithalamus, over the course of evolution the thalamus gained in importance. Originally, the thalamus represents an interface in the optic tract but as a result of phylogenetic development it increasingly became a polysensorial node (integration, modulation, coordination).
In this it comes to a progressive subdivision of the thalamus into specific areas that are connected with the striate bodies (see development of the telencephalon), the afferents (which, after a changeover, project to the primary cortical areas) and the cerebral cortices. The subthalamic nucleus regions (Luys' bodies) are to be found in the caudoventral region of the diencephalon and assigned to the switching nuclei of the basal ganglion systems.
The hypothalamus develops out of the subthalamic alar plate and from the base of the diencephalon (at most, some parts of the atrophying basal plate are also involved). Out of this emerge the hypothalamic nucleus region including the corpora mammillaria and the neurohypophysis. This nucleus region is in the service of steering vegetative functions (emotional states, eating behavior, sleep, bodily temperature, fluid balance, etc.).
The median ganglionic eminence
From the lateral wall of the alar plates the neuroblasts migrate into the white matter of the telencephalon and form there the median ganglionic eminence (future globus pallidus) as the only nucleus region of the basal ganglia that derive from the diencephalon.
The optic vesicle
At around the 19th day the ocular anlagen are already recognizable as two lateral bulges on the floor of the prospective prosencephalon. Since up to this point the anterior neuroporus is not yet closed, two corresponding optic sulci (foveolae opticae) appear on the inside of the neural groove. The development of the ocular anlagen is induced by the prechordal plate.
From the 25th day (stage 9), that is after the neural tube closes, the ocular anlagen widen to become the optic vesicles (vesiculae ophthalmicae), which remain connected with their area of origin, the future boundary between floor and side wall of the diencephalon, via the ocular peduncle (pedunculus ophthalmicus).