Development during the fetal phase
Over the course of the fetal phase the neopallium experiences considerable expansion at the expense of the archipallium and palaeopallium.
In this, the hemispheres gradually grow around the diencephalon. Through the strong proliferation of the amount of striate bodies, a thickening in the connection region between the telencephalon and diencephalon occurs so that these two parts merge into one another over large areas. The portion of the meninges that then still lie between the two brain sections atrophy and a fusion of the hemisphere floors with the thalamus takes place.
The broad connection zone permits the entry of nerve fibers that connect the hemispheres with the other segments of the nervous system. The white matter in the stem region appears as the internal capsule.
Out of the lateral neostriatum, lying on the floor, arise the caudate nucleus and the putamen in the course of the further development. In contrast, out of the medially lying palaeostriatum of the diencephalon emerges the globus pallidus (in short: pallidum). Macroscopically this, together with the putamen, is summarized as the lentiform (or lenticular) nucleus. The amygdala or archistriatum finally develops ventrally to the lentiform nucleus and apparently derives from the diencephalon and the telencephalon.