The next series of developments begins by examining two coronal sections through the diencephalon. The 1st section, shown in panel A to the right, shows a section through the diencephalon in the location where the cerebral hemispheres are growing laterally. Panel B to the right is a section slightly posterior to the 1st showing only the diencephalon.

Click the link to the right now to animate that animation. As the cerebral hemispheres continue to grow they expand superiorly, anteriorly, and posteriorly. These expanding hemispheres overgrow the diencephalon. However, note that the 3rd ventricle and lateral ventricles remain connected only at the interventricular foramina which remain as narrow connections located in the region from which the hemispheres first sprouted.

This is indicated in panel A to the right. As you can see in panel B to the right, the continually expanding hemispheres are growing against the diencephalon, covering its surface. Eventually, neural connections will be made between the diencephalon and the hemispheres in the region where this overgrowth occurs. Further expansion of the cerebral hemispheres is highlighted by continued expansion anteriorly to form the frontal lobes, and posteriorly to form the parietal and occipital lobes.

Furthermore, another outgrowth occurs from a portion of the posterior growth. This outgrowth projects first slightly inferiorly and then anteriorly to form the temporal lobe. Also note that the cerebellum is formed as an outgrowth of the alar plates of the pons.