Explanation

Diencephalon is a small, unpaired and median squarish part of forebrain. Its dorsal wall called epithalamus and the overlying pia-arachnoid matter are thrown into highly vascular internal folds or tufts invaginated into the diocoel. This dorsal wall is, therefore, called anterior choroid plexus. From the blood capillaries of this plexus some amount of plasma fluid continuously oozes out into the cerebrospinal fluid.