_a_, pedunculi cerebri; _b_, tractus transversus peduncularis; _c_, corpus geniculatum mediale; _d_, optic tract; _e_, optic chiasma; _f_, optic nerve; _g_, mammillary bodies; _h_, tuber cinereum; _i_, opening for infundibulum (which has been removed); _j_, posterior perforated area; _k_, pons. _III_, _V_, _VII_, _VIII_, third, fifth, seventh, and eighth cranial nerves.] C. PROSENCEPHALON.--The prosencephalon or primitive forebrain includes the =diencephalon= or ’tween-brain and the =telencephalon= or cerebral hemispheres. 4. _Diencephalon._--The diencephalon or ’tween-brain includes the =thalami= and the other parts bounding the third ventricle. The diencephalon is seen in entire brains only in ventral view (Fig. 138, _c_, _d_, _e_, etc.) The diencephalon may be considered as forming almost or quite the most cranial portion of the median nervous tube,--the cranial wall of the third ventricle (the =lamina terminalis=) (Fig. 143, _d_), ending in the median line in the deep fissure between the hemispheres of the cerebrum. Parts of the brain which extend farther craniad than this are lateral portions, due to the forward growth of the lateral hemispheres. The two hemispheres of the cerebrum may be considered as lateral outgrowths of the central ’tween-brain; these outgrowths have extended dorsad, laterad, craniad, and caudad, so as to cover almost completely the ’tween-brain. In early stages the cerebral hemispheres are projections from the _cranial end_ of the ’tween-brain, so that the plane of junction was nearly transverse, the cranial end of the ’tween-brain joining the caudal end of the hemispheres. With the increasing size and backward growth of the latter, the attachment to the ’tween-brain has been shifted from a cranial to nearly a lateral position, and at the same time the originally lateral surface of the ’tween-brain has become nearly caudal. This is shown in Fig. 141, the deep fissure at 1 marking the line of attachment between the ’tween-brain and the hemispheres. The dorsoventral plane of junction of ’tween-brain and hemispheres is (as Fig. 141 shows) not wholly lateral, but oblique, passing from its cranial end near the middle line caudolaterad.