Over the course of the evolution from lower to higher vertebrates the development of the telencephalon increases in importance while the brainstem experiences no profound phylogenetically-contingent alterations. The massive development of the cerebral hemispheres in human embryos, on the other hand, sums up, so to say, the phylogenesis.
The hemispheres expand laterally and caudally and thereby cover over the diencephalon, the dorsal part of the mesencephalon and a portion of the metencephalon. Nevertheless, the future hemispheres remain lifelong separated from each other by the medial sagittal fissure (fissura sagittalis cerebri).
On the telencephalon's side the two cerebral vesicles arise while the median lying lamina terminalis forms the rostral closure of the neural tube at the location of the earlier anterior neuroporus. In contrast to the cerebral vesicles the lamina terminalis develops further only negligibly.
The spreading of the cavity system in the hemisphere vesicles develops into the lateral ventricles that are connected with the IIIrd ventricle via the interventricular foramina (foramina Monroi).
Out of the roof of the hemisphere vesicles arises on all sides the cerebral envelope, the pallium, while its floor, the subpallium, thickens to become the ganglionic eminences (stage 15). From these the inner nucleus region of each hemisphere emerges later.
Corresponding to the enduring situation in lower vertebrates (lissencephalic) the surfaces of the hemisphere vesicles are initially smooth. From the 18th week, though, an enlargement and increase in the circumference of the pallium occurs that leads to the typical formation of cerebral windings (gyri) and furrows (sulci). When this process is completed, ca. two thirds of the cerebral surface lies hidden in the depths of the furrows.
Initially, the cerebral vesicles expand dorsally; afterward, they expand caudally and then become arch-shaped, ventrally to rostrally. The cerebral hemispheres thus exhibit a horseshoe-shaped curvature that is open rostro-ventrally. In the region of the center of curvature, an area, the insular cortex (or insula), remains retarded in its growth and is moved into the depths. Accordingly, the lateral sulcus (Sylvian fissure) comes to lie above this insular lobe. Going out from the lateral sulcus four cerebral lobes can be distinguished: the frontal, parietal, temporal and occipital lobes. From the 6th month the central sulcus (fissure of Rolando) and the parieto-occipital sulcus gradually begin to stand out as the boundaries among the corresponding lobes.