Third ventricle is derived from

C i.e. ThalamencephalonThe pa of the diencephalon of the brain that includes the thalamus, pineal gland, and adjacent structuresThalamencephalon also called thalamic region is a complex structure comprising thalamus (in the wider sense of the term thalamus, i.e. dorsal thalamus, or thalamus proper, plus subthalamus, or ventral thalamus) and several adjacent structures: epithalamus and metathalamusDEVELOPMENT:The cavity of the thalamencephalon opens behind into the cavity of the middle cerebral vesicle, and in front communicates with the hollow rudiments of the cerebral hemispheres, and eventually it becomes the cavity of the third ventricle.The floor of the thalamencephalon is ultimately developed into the optic chiasma, pa of the optic nerves, and the infundibulum.The latter comes in contact with a process from the mouth, uniting with which it ultimately forms the pituitary body.From the posterior pa of the roof of the thalamencephalon is developed the pineal gland .The anterior pa of the roof of the thalamencephalon becomes very thin, and its place is finally occupied by a thin membrane containing a vascular plexus, which persists in the roof of the third ventricle {choroidplexus).From the sides of the thalamencephalon, which become extremely thickened, are developed the optic thalami.