Development during the embryonic phase (first 8 weeks)
Over the course of the embryonic phase bulging occurs on the two cerebral vesicles on the sides of the telencephalon out of which the hemispheres develop. Their arched roofs form the cerebral covering, the pallium, while the floor, the subpallium, develops to become the striatum. Between the pallium and the subpallium lies the lateral ventricle.
From stage 14 (ca. 33 days) the cerebral hemispheres are recognizable as two lateral bulges of the telencephalon.
The neuroblasts of the ventricular zone emigrate from two regions and populate two portions of the hemispheres. Thus, in the pallium, gray matter arises as the cortex; in the subpallium gray matter is in the form of the ganglionic eminences that emerge from the striate bodies. In contrast to the pallium, which in this phase thickens only minimally, the subpallium swells considerably so that the ventricle space becomes increasingly constricted. Later, in the lower parts of the interhemispheric fissure (fissura longitudinalis cerebri) the commissures of the two hemispheres become connected with each other. The corpus callosum represents the mightiest commissure (see below: commissures of the telencephalon).
Finally, the two hemispheres meet in the median plane, remaining dorsally, though, separated from each other by the interhemispheric fissure (with the falx cerebri). The medially lying adhesion location of the hemisphere roofs at the diencephalon stay thin and epithelialial. In this a longitudinally running groove arises against the ventricular space, the choroid fissure, out of which the choroid plexus of the respective hemisphere arises in stage 19 – 23 (ca. 7th to 9th weeks). The neopallium greatly expands and pushes back ventrally the palaeopallium (olfactory bulb and tubercle) and dorsally the archipallium (hippocampus).
At the same time, through the animated mitosis activity in the ventricular zone, a thickening of the subpallium occurs.