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The Thalamencephalon is much reduced.

On the floor of the thalamencephalon the blind pouch-like infundibulum is in contact with the pituitary body, an outgrowth from the combined pituitary and olfactory pouch, which in the adult opens on to the top of the head just in front of the pineal area.

The original anterior vesicle from which the hemispheres evaginate is composed, as already shown, of an anterior part or telencephalon and a posterior or thalamencephalon; the whole forming the third ventricle in the adult.

In connexion with the thalamencephalon, though not really belonging to it, may be mentioned the anterior lobes of the pituitary body; these begin as an upward diverticulum from the posterior wall of the primitive pharynx or stomatodaeum about the fourth week.

From the ventral parts of these cerebral hemispheres the olfactory lobes are 403 constricted off, while just behind the openings of the foramina of Munro a constriction occurs which divides the prosencephalon into two secondary vesicles, the anterior of which, containing the foramina of Munro, is called the telencephalon, while the posterior is the thalamencephalon or diencephalon.